apierr: Alias ErrResourceAlreadyExists to ErrAlreadyExists (#1639)

## Changes

`apierr` defined two 409 sentinels with identical messages but distinct
pointers:

- `ErrAlreadyExists` — mapped from `error_code: ALREADY_EXISTS`
(gRPC canonical)
- `ErrResourceAlreadyExists` — mapped from `error_code:
RESOURCE_ALREADY_EXISTS` (Databricks-specific)

Consumers using `errors.Is` with one sentinel did not match errors
wrapped with the other, even though both represent the same class of 409
conflict. Which of the two `error_code` strings the backend returns is
not guaranteed, so consumers had to either assert the generic
`ErrResourceConflict` or duplicate both sentinels in their checks.

This came up downstream in databricks/cli#5043
where a `TestDashboardAssumptions_WorkspaceImport` assertion on
`ErrResourceAlreadyExists` started failing because the Lakeview API
returns `ALREADY_EXISTS` rather than `RESOURCE_ALREADY_EXISTS`.

Alias `ErrResourceAlreadyExists` to `ErrAlreadyExists` so `errors.Is`
matches either `error_code` regardless of which the backend emits. The
exported symbol is preserved, so this is source- and binary-compatible
for all existing callers.

Also extended `TestErrorMapping` to cover both error_codes against both
sentinels and against the shared `ErrResourceConflict` parent.

## Tests

- [x] `go test ./apierr/...` passes, including new
`RESOURCE_ALREADY_EXISTS` ↔ `ErrAlreadyExists` and `ALREADY_EXISTS` ↔
`ErrResourceAlreadyExists` cases.
- [x] `make fmt` clean.
- [x] `make lint` clean.

Signed-off-by: simon <simon.faltum@databricks.com>

Author: simonfaltum

NEXT_CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bug Fixes
 
+ * Alias `apierr.ErrResourceAlreadyExists` to `apierr.ErrAlreadyExists` so `errors.Is` matches both the gRPC-canonical `ALREADY_EXISTS` and the Databricks-specific `RESOURCE_ALREADY_EXISTS` error codes, regardless of which one the backend returns.
+
 ### Documentation
 
 ### Internal Changes

apierr/error_mapping.go

@@ -17,7 +17,11 @@ var testTable = []mapping{
 	codeMapping(409, "ABORTED", ErrAborted),
 	codeMapping(409, "ABORTED", ErrResourceConflict),
 	codeMapping(409, "ALREADY_EXISTS", ErrAlreadyExists),
+	codeMapping(409, "ALREADY_EXISTS", ErrResourceAlreadyExists),
 	codeMapping(409, "ALREADY_EXISTS", ErrResourceConflict),
+	codeMapping(409, "RESOURCE_ALREADY_EXISTS", ErrAlreadyExists),
+	codeMapping(409, "RESOURCE_ALREADY_EXISTS", ErrResourceAlreadyExists),
+	codeMapping(409, "RESOURCE_ALREADY_EXISTS", ErrResourceConflict),
 	statusMapping(429, ErrTooManyRequests),
 	codeMapping(429, "REQUEST_LIMIT_EXCEEDED", ErrTooManyRequests),
 	codeMapping(429, "REQUEST_LIMIT_EXCEEDED", ErrRequestLimitExceeded),