test: Add httpx dependency to mcp and fastmcp tests

[alexander-alderman-webb]

Description

The MCP test suite relies on the httpx package due to some fixtures in conftest.py.

The package switched to httpx2 as part of its v2 major, so httpx is no longer pulled in transitively.

Issues

Closes #6899

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Codecov Results 📊

✅ 98541 passed | ⏭️ 6772 skipped | Total: 105313 | Pass Rate: 93.57% | Execution Time: 357m 3s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -169
Passed Tests	📉 -134
Failed Tests	📉 -2
Skipped Tests	📉 -33

➖ Removed Tests (1)

View removed tests

• test_circular_references_span_streaming
  • File: tests.tracing.test_misc

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2484 uncovered lines.
✅ Project coverage is 89.87%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.57%    89.87%     +0.3%
==========================================
  Files          193       193         —
  Lines        24514     24514         —
  Branches      8676      8676         —
==========================================
+ Hits         21956     22030       +74
- Misses        2558      2484       -74
- Partials      1400      1393        -7

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