Upgrade tree-sitter-language-pack to 1.9.0 (breaks Dart and R parsers)

[GoodbyePlanet]

Summary

tree-sitter-language-pack is currently held back at >=1.5.0,<1.6.3 (resolved to 1.6.2). The bump to 1.9.0 was excluded from #52 because it breaks the Dart and R parsers. This issue tracks doing the upgrade properly in a dedicated PR.

The problem

In 1.9.0, tree-sitter-language-pack drops its explicit tree-sitter dependency and bundles a newer tree-sitter core. The newer core's parse() no longer accepts a bytes object the way our code calls it, producing:

TypeError: 'bytes' object is not an instance of 'str'
while processing 'source'

This only affects parsers that get their grammar from the language pack — there are no standalone tree-sitter-dart / tree-sitter-r packages in our deps — which is why the other 22 parsers (using their own tree-sitter-<lang> packages) are unaffected.

Affected code

• server/parser/dart.py:285 — tree = self._parser.parse(source)
• server/parser/r.py:126 — tree = self._parser.parse(source)

Both obtain their parser via from tree_sitter_language_pack import get_parser.

Failing tests

tests/parser/test_dart.py::test_empty_file_returns_no_symbols
tests/parser/test_dart.py::test_canonical_user_widget_fixture
tests/parser/test_r.py::test_empty_file_returns_no_symbols
tests/parser/test_r.py::test_canonical_utils_fixture

(Observed in CI on #52: 4 failed, 200 passed.)

Proposed fix

Adapt the parse() calls in dart.py and r.py to the newer tree-sitter API — likely passing str (or using the encoding= / read-callable form) instead of raw bytes for the language-pack-backed parsers. Verify whether the standalone tree-sitter-<lang> parsers need the same treatment once the bundled core is upgraded, then:

1. Bump tree-sitter-language-pack to 1.9.0 in pyproject.toml and re-lock.
2. Update the Dart and R parsers to the new parse() API.
3. Confirm uv run pytest is fully green.

Context

• Held back in chore(deps): Bump the dependencies group across 1 directory with 5 updates #52 (manual commit on the Dependabot branch).