fix(regex): reject empty unicode brace escape

[hackwaly]

This PR fix wrong behavior of regex parser, make it reject empty unicode brace escape.

[Copilot]

Pull request overview

This PR tightens the regex parser’s escape handling so that an empty Unicode brace escape (\u{}) is rejected as an invalid escape sequence, rather than being partially parsed as literal characters (notably inside character classes).

Changes:

• Reject stray \u prefixes in both character-class parsing (class_atom) and general term parsing (term) by raising HINT_INVALID_ESCAPE when they don’t match a valid \uXXXX or \u{X...} form.
• Add a regression test ensuring [\u{}] fails with ParserError(..., hint="Invalid escape sequence").

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
string/internal/regex_parser/parser.mbt	Adds explicit rejection of invalid/partial \u escape sequences to prevent \u{} from being mis-parsed.
string/internal/regex_parser/parser_test.mbt	Adds a test case covering the newly rejected empty Unicode brace escape inside a character class.

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[coveralls]

Coverage Report for CI Build 6165

Coverage decreased (-0.004%) to 90.682%

Details

• Coverage decreased (-0.004%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (1 of 2 lines covered, 50.0%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
string/internal/regex_parser/parser.mbt	2	1	50.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	17900
Covered Lines:	16232
Line Coverage:	90.68%
Coverage Strength:	332092.66 hits per line

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💛 - Coveralls