Add --search-type flag for semantic and hybrid issue search - #14006

Adds a --search-type <lexical|semantic|hybrid> flag to gh search issues, defaulting to lexical so existing behavior is unchanged. When set to semantic or hybrid, the command sends the REST search_type query parameter.

Behavior

• Semantic/hybrid are scoped to issues, so --include-prs cannot be combined with them, --sort / --order are rejected (results are relevance-ranked), and --web is rejected (the web search UI cannot carry search_type).
• Unknown values are rejected client-side, since the API silently ignores them.
• Support is feature-detected via the existing SearchType enum introspection (ISSUE_SEMANTIC / ISSUE_HYBRID), present on github.com and ghe.com, absent on single-tenant GHES, where the command fails with a clear "not supported on this host" error.
• Semantic/hybrid use a separate, smaller rate-limit bucket, so fetching is bounded to a single page.

Testing

• Unit tests cover query encoding, the searcher request (param present for semantic/hybrid, absent for lexical, single-page bound, and the unsupported-host error), feature detection, and flag validation.
• Manually verified on github.com that --search-type semantic|hybrid sends the parameter and that no flag sends none.

Notes

• search_type is independent of advanced_search=true.
• Semantic ranking applies to keyword queries, quoted exact phrase queries are handled as lexical by the backend.