Add tests for proxy.restBackendCaller.CallTool

Cover the pull_request_read, issue_read, search_repositories, and
unsupported-tool branches of CallTool which had no tests.

Test cases added:
- pull_request_read: success with pullNumber as string, as float64
  (JSON number), missing owner/repo/pullNumber, 404 from upstream
- issue_read: success with issue_number as string, as float64,
  missing owner/repo/issue_number, 404 from upstream
- search_repositories: success with default perPage, custom perPage
  as float64, missing/empty query, 422 from upstream, query forwarding
- unsupported tool names: all return "unsupported tool" error
- invalid args type (non-map): all tool branches return "unexpected args type"
- request header: server token preferred over client auth, fallback to
  client auth when no server token configured
- response format: MCP content envelope wraps raw GitHub API response body

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]