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 #4332

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