feat: Add search_type support to issue search

[kelsey-myers]

Fixes #4415

The search issues and pull requests endpoint supports a search_type query parameter (semantic or hybrid, defaulting to lexical), and returns the type that actually ran in the response body. Neither is currently exposed, so Search.Issues can only do lexical search, and callers can't tell when the API has fallen back to lexical for an ineligible query.

Adds SearchType to SearchOptions (mirroring AdvancedSearch) and to IssuesSearchResult.

I'm updating the GitHub MCP server's search_issues tool to support semantic search, and we'd like to use these rather than wrapping the HTTP transport to rewrite the query string and re-parse the response body.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (0eab779) to head (7bdbd06).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4414   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         194      194           
  Lines       19765    19765           
=======================================
  Hits        19279    19279           
  Misses        268      268           
  Partials      218      218           

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[gmlewis]

Thank you, @kelsey-myers!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra