Test/57 add mock GitHub api server

[TanishaD111]

Summary

This PR closes issue #57. The GitHubTool in agent/tools/github_tool.py calls the live GitHub REST API (https://api.github.com) to fetch repository metadata, so any integration test that exercises it needs real network access and is subject to GitHub's rate limits and auth. As a result, those tests are skipped in CI, there is currently no tests/integration/test_github_tool.py and no tests/fixtures/github_responses/ directory, leaving the tool's request handling and error paths (404, 403/rate limit) untested by automation. The fix is to stand up a lightweight local mock HTTP server (pytest-httpserver) that serves canned JSON fixtures for GitHub endpoints and to point the tool's base_url at it, so the GitHub tool tests can run deterministically and offline in CI without hitting the real API.

Issue

Closes #57

Changes

• agent/tools/github_tool.py: added an optional base_url parameter to init (defaulting to a new DEFAULT_BASE_URL constant)
• tests/fixtures/github_responses/: 4 canned responses: a fully-populated repo, one with explicit null description/language/homepage, and real GitHub 404 and 403 error bodies
• tests/integration/test_github_tool.py: 10 tests / 14 cases covering 200 (full field mapping, asserted as one dict equality), 404, 403, 500, null-field coalescing, Authorization header forwarding, has_readme reflecting the HEAD /readme result, and missing/empty/None input across 5 parametrized shapes. Marked pytest.mark.integration

Testing

• Unit tests pass (make test-unit)
• Integration tests pass (make test-integration)
• Linter passes (make lint)
• Type checker passes (make typecheck)
• New/updated tests cover the changes

Some of these errors are not from my files. The make test-integration passes all 14 tests for my issue.

Screenshots / Demo

n/a

Notes for Reviewers

n/a