Support quoted literal search terms for CLI flags (e.g. --dry-run) (#14041)

[chishxd]

Purpose

Fixes search for CLI-flag-style terms (e.g. --dry-run, -v) which are currently
unsearchable due to two separate issues in the search pipeline:

1. Indexer : _word_re in sphinx/search/__init__.py only matched \w+,
   so hyphens were treated as word separators at index-build time. A page mentioning
   --dry-run would index the term as dry and run — the flag itself was never
   stored as a searchable token.

2. Query parser : searchtools.js's splitQuery/_parseQuery treats any
   term beginning with - as an exclusion operator. Even if the index preserved the
   token, searching --dry-run (or -v) would silently exclude results rather than
   search for them.

The simple fix:

• _word_re is updated to (?<!\w)-{1,2}\w[\w-]*|\w+ which preserves leading
  single/double hyphens as part of a token only at a word boundary

• splitQuery now extracts "quoted phrases" from the query string before running
  the normal tokenizer on the remainder, returning {quotedTerms, plainTerms}
  instead of a flat array. _parseQuery adds quoted terms directly to searchTerms,
  bypassing the - exclusion check entirely

Scope / known limitations:

• This does not change behavior for unquoted CLI-flag searches... typing --dry-run
  without quotes still goes through the normal splitter/exclusion path. Quoting is
  the documented workaround, matching the direction discussed in the linked issue.
• SearchLanguage.js_splitter_code, the existing hook that lets custom language
  search implementations override splitQuery, is not yet updated to the new
  {quotedTerms, plainTerms} return shape. A custom override returning a flat array
  would silently break with this change. I haven't touched other-language search
  implementations, flagging this for maintainer input on whether it needs handling
  in this PR or a follow-up.

Tests added:

• Python: unit tests for _word_re covering CLI flags, short flags, and the
  compound-word regression guard.
• JS: unit tests for splitQuery (quoted extraction, unchanged plain-term behavior)
  and _parseQuery (quoted terms bypass exclusion).
• Integration: a sphinx-build fixture asserting --dry-run appears as an indexed
  term in the built searchindex.js, and that a quoted search for it returns results.

CHANGES.rst entry added.

References

• Fixes Make CLI documentation searchable: Enable search for --options #14041
• Related: Word exclusion breaks search #13892

AI Disclosure

AI tools were used during preparation of this PR:

• I used Claude for design discussion and planning.. mostly like a rubber duck, working through the
  distinction between the indexer-side and query-parser-side problems, reviewing the
  regex design for edge cases (compound words, word-boundary detection), and
  reviewing/debugging the JS diff stuff
• Also used GLM with opencode for test suite...

All code in this PR was reviewed and understood by me before submission, and all
interaction with maintainers/reviewers on this PR will be done by me directly, per
the project's AI policy.

[chishxd]

woahh that's some crazy amount of Red crosses... But i also tested running test on clean master branch, and got these same errors, the Pygments update seems to have broken test assertions... And Maybe I messed up some linting error, for which I will push some update soon