Allow selecting fields in pull request file responses

[adbcodes]

Summary

Adds optional fields selection to pull_request_read with method: "get_files", allowing callers to request filenames or metadata without patches. Omitting fields or passing an empty array preserves the existing response.

Why

Callers needing only changed filenames currently receive potentially large patches, consuming unnecessary model context.
Fixes #3236

What changed

• Added optional fields to the tool schema using existing field-selection helpers.
• Added regression tests and updated the tool schema snapshot and README.

MCP impact

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

Added optional fields for get_files. Existing calls continue returning the full response.

Prompts tested (tool changes only)

Tested through Codex CLI connected to the fixed local MCP server, using github/github-mcp-server PR #2666:

• “List only the filenames changed.”
  Selected filename only; response contained no patches.
• “Show the changed filenames and their statuses. Omit patches.”
  Selected filename and status only.
• “Show the filenames and patches.”
  Selected filename and patch; patches were returned.

Verified the generated tool arguments and raw MCP responses.

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)