fix(extgen): extract Go function bodies via go/ast instead of brace counting

[alexandre-daubois]

No description provided.

[dunglas]

Needs a rebase

[Copilot]

Pull request overview

This PR updates the internal/extgen generator’s Go source extraction logic to use go/ast parsing rather than manual brace counting, making function/method body extraction resilient to braces appearing in strings and comments.

Changes:

• Replace line-scanning + brace counting in FuncParser with go/parser + AST-based source slicing.
• Replace line-scanning + brace counting in class method parsing with AST-based extraction.
• Add shared AST utilities and a regression test covering braces inside strings/comments.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
internal/extgen/funcparser.go	Parse Go files via go/parser, associate //export_php:function directives via AST doc comments, and extract function source via node offsets.
internal/extgen/funcparser_test.go	Adds a regression test ensuring braces in strings/comments don’t break extraction.
internal/extgen/classparser.go	Migrates method extraction to AST-based parsing and removes brace-counting scanner logic.
internal/extgen/astutil.go	Adds reusable helpers for directive lookup, node source extraction, and orphan-directive detection.

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