Fix blackd error handling: split SourceASTParseError from ASTSafetyError

[Bahtya]

Summary

Per @JelleZijlstra's review feedback on #5074, this PR splits the error handling instead of treating all ASTSafetyError as 400.

Changes

1. New exception: SourceASTParseError (src/black/parsing.py)

   • Raised when the source file cannot be parsed by ast.parse() but was accepted by Black's more lenient lib2to3 parser
   • This is a user input issue, not a Black bug

2. Updated assert_equivalent (src/black/__init__.py)

   • Source AST parse failure → SourceASTParseError (user input issue)
   • Dest AST parse failure → ASTSafetyError (Black bug → 500)
   • Non-equivalent output → ASTSafetyError (Black bug → 500)

3. Updated blackd (src/blackd/__init__.py)

   • SourceASTParseError → 400 Bad Request
   • ASTSafetyError → 500 Internal Server Error (unchanged, caught by generic Exception handler)

Closes

Fixes #3616

Analysis of all ASTSafetyError uses

Location	Cause	Error type	blackd status
assert_equivalent src AST parse fail	User input (lenient parser)	SourceASTParseError	400
assert_equivalent dest AST parse fail	Black bug	ASTSafetyError	500
assert_equivalent src ≠ dst	Black bug	ASTSafetyError	500
check_stability_and_equivalence re-raise	Depends on underlying	Propagated	400 or 500

[Bahtya]

Hi team, just wanted to follow up on this PR. Would appreciate any feedback!

[cobaltt7]

The code looks good to me, could you just add changelog entries (one in the Output section about the new error, and one in the Blackd section about the corrected handling) and updated tests?

[github-actions]

diff-shades results comparing this PR (1a6e264) to main (e079b7e):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[JelleZijlstra]

Looks good, but please add a changelog entry.

[Bahtya]

Thanks @cobaltt7 @JelleZijlstra for the review! I will add the changelog entries (Output section for the new error, Blackd section for the corrected handling) and update the tests. Pushing changes shortly.