fix: restore lexical search semantics for search_issues

[lorenzozanee]

Summary

Restore lexical GitHub issues search for search_issues so keyword and qualifier queries return the expected results.

Why

Fixes #3188

What changed

• Route hosted search_issues requests through GitHub's lexical Issues Search API.
• Update the tool description/schema snapshot and regression coverage.
• Keep the existing response and pagination behavior, including incomplete_results.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• "Find open issues in modelcontextprotocol/python-sdk containing the keyword transport"

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)