fix(python): emit wildcard default for union or-patterns (#4649)

[dbrattli]

Fixes #4649

Problem

Matching against multiple union cases in an or-pattern returned None (Python) for one of the cases:

type Case = BestCase | MidCase | WorstCase

let atLeastMid (c: Case) =
    match c with
    | BestCase | MidCase -> true
    | WorstCase -> false

transpiled to (tag 0 missing):

def at_least_mid(c: Case) -> bool:
    match c.tag:
        case 1:
            return True
        case 2:
            return False

so atLeastMid BestCase fell through the match and returned None instead of True.

Root cause

In transformSwitchPatternAsMatch, FCS lowered the match so BestCase (tag 0) became the default branch, sharing its target body (return True) with MidCase. The generator had a defaultAlreadyCovered check that dropped the wildcard case _: whenever the default's target index was already used by an explicit case — leaving the tag only reachable via the default (tag 0) unmatched.

Fix

Always emit the wildcard default (it's the only thing covering subject values not listed explicitly). Instead, drop the redundant explicit cases that route to the default's target, since the wildcard already covers them. The example now generates:

def at_least_mid(c: Case) -> bool:
    match c.tag:
        case 2:
            return False
        case _:
            return True

Testing

• Added regression test test or-pattern over union cases sharing default target in tests/Python/TestPatternMatch.fs.
• Full Python suite passes (2354 tests).
• No new Pyright type-check errors.

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded