Changes

refactor(flaky-tests): simplify GitHub types and API usage, remove custom wrappers (#4567)

## Summary

Simplify GitHub-related code in `flaky-tests`: remove custom wrapper
layer and use Octokit types/APIs directly.

### Changes

1. **Remove custom `GitHubClient` wrapper class** (~120 lines)  
`GitHubClient` wrapped 5 Octokit API methods (`searchIssues`,
`createIssue`, `reopenIssue`, `addLabels`, `addComment`), each manually
mapping response fields to a custom `GitHubIssueApi` interface. Now
calls Octokit's native REST APIs directly.

2. **Remove `GitHubIssueApi` and `OctokitLike` interfaces**  
No more custom interfaces — use `Octokit` type directly from
`@octokit/rest`.

3. **Flatten helper functions**  
Extract Octokit calls into module-level standalone functions
(`searchIssues`, `createIssue`, etc.) instead of class instance methods.

4. **Test at Octokit mock level**  
Tests no longer mock `GitHubClient`. Instead, inject a mock Octokit
instance via the `octokit` constructor option.

5. **Fix GitHub API deprecation warning**  
Add `X-GitHub-Api-Version: 2026-03-10` header to use the latest API
version.

### Files Changed

- `tools/reporters/ci/flaky-tests/core/GithubIssueManager.ts` — major
simplification
- `tools/reporters/ci/flaky-tests/core/GithubIssueManager.test.ts` —
updated mocking strategy
- `tools/reporters/ci/flaky-tests/deno.json` / `deno.lock` — dependency
updates
- `tools/reporters/ci/flaky-tests/render/HtmlRenderer.ts` — label text
optimization

---------

Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
 (commit: 8369115)