fix(ast-engine): harden Language::from_path default — better diagnostics and contract tests

[Copilot]

The previous unimplemented!("from_path is not implemented") message gave no indication of which type failed or how to fix it. The doc comment still described the old None-returning behavior.

Changes

• Panic message: includes the concrete type name and remediation guidance via std::any::type_name::<Self>()

  unimplemented!(
      "Language::from_path is not implemented for type `{}`. \
       Override Language::from_path for this type if path-based detection is required.",
      std::any::type_name::<Self>()
  )

• Doc comment: updated to reflect that the default panics; implementors must return Some(Self) / None
• Tests: two new unit tests pin the contract
  • default_from_path_panics — asserts #[should_panic(expected = "Language::from_path is not implemented for type")] on a bare impl
  • overridden_from_path_does_not_panic — verifies an overriding impl returns Some/None correctly
• Test fixtures: extracted shared TSX helpers (tsx_kind_to_id, tsx_field_to_id, tsx_ts_language) to remove duplication across test Language impls

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[github-actions]

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