fix: file_api/ast/metadata bug fixes and simplifications

[henryiii]

Autogenerated from #1317.

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This fixes a reviewed batch of bugs and simplifications in file_api/, ast/,
and metadata/, each with a regression test. All findings were verified against
the code before fixing; none were skipped.

Fixes

1. metadata/regex.py + metadata/template.py — wrong superset operator.
   if settings.keys() > KEYS: is a proper-superset check, so it only fired
   when the user supplied all valid keys plus an extra; a typo'd key (e.g.
   removes) passed silently and was ignored. Changed to
   if settings.keys() - KEYS: in both. Added regression tests that a bogus key
   raises.

2. file_api/reply.py + file_api/_cattrs_converter.py — toolchains
   jsonFile never followed. The toolchains-v1 reply is requested and
   modeled, but make_class/the cattrs converter only followed jsonFile for
   {CodeModel, Target, Cache, CMakeFiles}. Toolchains was missing, so the
   populated toolchains-v1-*.json was never read and silently structured into
   an empty Toolchains stub. Added Toolchains to the jsonFile-following set
   in both converters; strengthened test_included_dir to assert the actual
   toolchain content loads (and that both converters agree).

3. file_api/model/codemodel.py — Link.commandFragments had no default.
   Per the CMake File API spec it's optional (sibling Archive.commandFragments
   already defaults), so a target whose link object omitted it failed the whole
   codemodel conversion. Gave it the same default_factory=list. Added a test
   structuring a minimal link dict without commandFragments.

4. ast/ast.py — comments leaked into argument values. The
   argument-accumulation loop appended COMMENT / BRACKET_COMMENT token values,
   so cmake_minimum_required(VERSION 3.15 # why not\n) yielded value
   "3.15 # why not", which flowed into Version(...)/SpecifierSet(...) and
   raised uncaught InvalidVersion during settings parsing. Comment tokens are
   now skipped; since a bracket comment's regex also swallows leading
   whitespace, it is replaced with a single space to keep tokens separated.
   Added find_min_cmake_version regression tests with in-parens comments
   (including a leading bracket comment).

5. ast/ast.py + settings/skbuild_read_settings.py — opaque traceback on
   malformed CMakeLists. An unterminated block made contents[-1] raise a
   bare IndexError, and the missing-open-paren case raised AssertionError;
   the call site only caught FileNotFoundError, so a CMakeLists.txt the parser
   chokes on produced an opaque traceback instead of the graceful fallback the
   surrounding code anticipates. Added a dedicated ParseError raised by the
   ast module and caught at the find_min_cmake_version call site, which now
   falls through to the existing warning path. Added a settings test.

6. metadata/__init__.py — authors/maintainers validation crashed on bad
   input. The _LIST_DICT_FIELDS check called .items() on each list element
   before checking it was a dict, so ["string"] raised AttributeError
   instead of the intended RuntimeError; the message also said "dictionary of
   strings" but should be "list of dictionaries of strings". Both fixed. Also
   tightened the optional-dependencies check to verify list elements are
   strings. Added regression tests.

7. ast/tokenizer.py — dead LEGACY regex alternative removed. The second
   alternative (|"(?:[^"\\]*(?:\\.[^"\\]*)*)*") was unreachable: the preceding
   QUOTED alternative matches a strict superset at every position. Removed it;
   ast tests confirm no behavior change.

8. file_api/reply.py — simplified if origin is not None and origin is list: to if origin is list:.

Tests

uv run pytest tests/test_cmake_ast.py tests/test_fileapi.py tests/test_dynamic_metadata_unit.py tests/test_skbuild_settings.py tests/test_dynamic_metadata.py tests/test_auto.py -q -n auto → 131 passed.

prek -a --quiet: ruff, ruff-format, and mypy pass. (The check-sdist hook
flags only pre-existing untracked uv.lock / .claude/settings.local.json
worktree artifacts, unrelated to this change.)

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