Add TEXTJOIN function with i18n and documentation (#1640)

## Summary

Adds the `TEXTJOIN` function — joins text from multiple strings and/or
ranges with a configurable delimiter.

Replaces #1625 (was opened from fork, now from upstream branch
directly).

### Features
- Scalar and array/range delimiter support with cycling behavior
- `ignore_empty` parameter to skip empty strings
- Type coercion (numbers, booleans → strings)
- Error propagation from both delimiter and text arguments
- 32,767 character limit (Excel compatibility)
- i18n translations for all 17 supported languages
- Documentation in `built-in-functions.md`

### Implementation
- New `textjoin` method + `flattenArgToStrings` helper in `TextPlugin`
- `repeatLastArgs: 1` metadata pattern (same as SUMPRODUCT, etc.)
- Defensive `CellError` check on `coerceScalarToString` return value

### Changed files
| File | Change |
|------|--------|
| `src/interpreter/plugin/TextPlugin.ts` | `textjoin()` +
`flattenArgToStrings()` |
| `src/error-message.ts` | `TextJoinResultTooLong` message |
| `src/i18n/languages/*.ts` (17 files) | TEXTJOIN translations |
| `docs/guide/built-in-functions.md` | TEXTJOIN row (alphabetically
between TEXT and TRIM) |

### Review feedback addressed (from #1625)
- Tests moved to private `hyperformula-tests` repo (companion PR
pending)
- Fixed docs alphabetical ordering
- Fixed unsafe `as string` cast in `flattenArgToStrings`

## Test plan

- [x] 35 tests in
`hyperformula-tests/unit/interpreter/function-textjoin.spec.ts`
- [x] Full suite: 480 suites / 5396 tests passed

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Adds a new interpreter function (`TEXTJOIN`) with range flattening and
type coercion, which touches formula evaluation paths and may introduce
edge-case regressions around error propagation and large-string
handling.
> 
> **Overview**
> Adds the new `TEXTJOIN` spreadsheet function, including interpreter
support for joining scalars and ranges with a delimiter (including
delimiter cycling), optional skipping of empty strings, and consistent
error propagation.
> 
> Introduces a new `ErrorMessage.ResultTooLong` and enforces Excel’s
32,767-character output limit (returning `#VALUE!` when exceeded).
> 
> Updates function documentation, the unreleased changelog, and adds
`TEXTJOIN` translations across all supported language packs.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
81426d7. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Kuba Sekowski <jakub.sekowski@handsontable.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

CHANGELOG.md

@@ -7,6 +7,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Added
+
+- Added a new function: TEXTJOIN. [#1640](https://github.com/handsontable/hyperformula/pull/1640)
+
 ### Fixed
 
 - Fixed the IRR function returning `#NUM!` error when the initial investment significantly exceeds the sum of returns. [#1628](https://github.com/handsontable/hyperformula/issues/1628)

docs/guide/built-in-functions.md

@@ -502,6 +502,7 @@ Total number of functions: **{{ $page.functionsCount }}**
 | SUBSTITUTE  | Returns string where occurrences of Old_text are replaced by New_text. Replaces only specific occurrence if last parameter is provided.                                                                                                                                                                                         | SUBSTITUTE(Text, Old_text, New_text, [Occurrence]) |
 | T           | Returns text if given value is text, empty string otherwise.                                                                                                                                                                                                                                                                    | T(Value)                                           |
 | TEXT        | Converts a number into text according to a given format.<br>By default, accepts the same formats that can be passed to the [`dateFormats`](../api/interfaces/configparams.md#dateformats) option, but can be further customized with the [`stringifyDateTime`](../api/interfaces/configparams.md#stringifydatetime) option. | TEXT(Number, Format)                               |
+| TEXTJOIN    | Joins text from multiple strings and/or ranges with a delimiter. Supports array/range delimiters that cycle through gaps. When ignore_empty is TRUE, empty strings are skipped. Returns #VALUE! if result exceeds 32,767 characters.                                                                                                 | TEXTJOIN(Delimiter, Ignore_empty, Text1, [Text2, ...]) |
 | TRIM        | Strips extra spaces from text.                                                                                                                                                                                                                                                                                                  | TRIM("Text")                                       |
 | UNICHAR     | Returns the character created by using provided code point.                                                                                                                                                                                                                                                                     | UNICHAR(Number)                                    |
 | UNICODE     | Returns the Unicode code point of a first character of a text.                                                                                                                                                                                                                                                                  | UNICODE(Text)                                      |

src/error-message.ts

@@ -73,6 +73,7 @@ export class ErrorMessage {
   public static ComplexNumberExpected = 'Complex number expected.'
   public static ShouldBeIorJ = 'Should be \'i\' or \'j\'.'
   public static SizeMismatch = 'Array dimensions mismatched.'
+  public static ResultTooLong = 'Result exceeds the maximum allowed length.'
   public static FunctionName = (arg: string) => `Function name ${arg} not recognized.`
   public static NamedExpressionName = (arg: string) => `Named expression ${arg} not recognized.`
   public static LicenseKey = (arg: string) => `License key is ${arg}.`

src/i18n/languages/csCZ.ts

@@ -215,6 +215,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'TBILLPRICE',
     TBILLYIELD: 'TBILLYIELD',
     TEXT: 'HODNOTA.NA.TEXT',
+    TEXTJOIN: 'TEXTJOIN',
     TIME: 'ČAS',
     TIMEVALUE: 'ČASHODN',
     TODAY: 'DNES',

src/i18n/languages/daDK.ts

@@ -215,6 +215,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'STATSOBLIGATION.KURS',
     TBILLYIELD: 'STATSOBLIGATION.AFKAST',
     TEXT: 'TEKST',
+    TEXTJOIN: 'TEKST.KOMBINER',
     TIME: 'TID',
     TIMEVALUE: 'TIDSVÆRDI',
     TODAY: 'IDAG',

src/i18n/languages/deDE.ts

@@ -215,6 +215,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'TBILLKURS',
     TBILLYIELD: 'TBILLRENDITE',
     TEXT: 'TEXT',
+    TEXTJOIN: 'TEXTVERKETTEN',
     TIME: 'ZEIT',
     TIMEVALUE: 'ZEITWERT',
     TODAY: 'HEUTE',

src/i18n/languages/enGB.ts

@@ -217,6 +217,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'TBILLPRICE',
     TBILLYIELD: 'TBILLYIELD',
     TEXT: 'TEXT',
+    TEXTJOIN: 'TEXTJOIN',
     TIME: 'TIME',
     TIMEVALUE: 'TIMEVALUE',
     TODAY: 'TODAY',

src/i18n/languages/esES.ts

@@ -215,6 +215,7 @@ export const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'LETRA.DE.TES.PRECIO',
     TBILLYIELD: 'LETRA.DE.TES.RENDTO',
     TEXT: 'TEXTO',
+    TEXTJOIN: 'UNIRCADENAS',
     TIME: 'NSHORA',
     TIMEVALUE: 'HORANUMERO',
     TODAY: 'HOY',

src/i18n/languages/fiFI.ts

@@ -215,6 +215,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'OBLIG.HINTA',
     TBILLYIELD: 'OBLIG.TUOTTO',
     TEXT: 'TEKSTI',
+    TEXTJOIN: 'TEKSTI.YHDISTÄ',
     TIME: 'AIKA',
     TIMEVALUE: 'AIKA_ARVO',
     TODAY: 'TÄMÄ.PÄIVÄ',

src/i18n/languages/frFR.ts

@@ -215,6 +215,7 @@ const dictionary: RawTranslationPackage = {
     TBILLPRICE: 'PRIX.BON.TRESOR',
     TBILLYIELD: 'RENDEMENT.BON.TRESOR',
     TEXT: 'TEXTE',
+    TEXTJOIN: 'JOINDRE.TEXTE',
     TIME: 'TEMPS',
     TIMEVALUE: 'TEMPSVAL',
     TODAY: 'AUJOURDHUI',