Add remaining market data test coverage from review follow-ups

[jkirkmannz]

• Add SSE integration tests for stream.py by driving _generate_events directly with a fake Request, since both httpx.ASGITransport and FastAPI's TestClient deadlock against this endpoint's unbounded while-True/disconnect-driven loop (confirmed via faulthandler). Coverage for stream.py goes from 31% to 94%.
• Add a PriceCache concurrent-writers test (8 threads x 200 updates) asserting the version counter exactly matches, to catch lost updates under real thread interleaving.
• Add a full 10-ticker GBMSimulator test covering the mixed tech/finance/TSLA correlation matrix, not just the 1-2 ticker cases.
• Add httpx as a dev dependency for ASGI testing.

Overall coverage: 91% -> 97%, 84/84 tests passing.

Claude-Session: https://claude.ai/code/session_011jXykWYaYoRkBpojjASfru

[Copilot]

🟢 Approval recommended

No unresolved review comments or approval blockers were identified.

Pull request overview

Adds remaining market-data test coverage, raising reported coverage from 91% to 97%.

Changes:

• Added SSE router and event-generation tests.
• Added concurrent PriceCache and full 10-ticker simulator tests.
• Added and locked httpx as a development dependency.

File summaries

File	Description
planning/MARKET_DATA_REVIEW.md	Documents coverage and verification results.
backend/uv.lock	Locks httpx and transitive packages.
backend/tests/market/test_stream.py	Tests SSE routing and event generation.
backend/tests/market/test_simulator.py	Tests the full ticker set.
backend/tests/market/test_cache.py	Tests concurrent cache updates.
backend/pyproject.toml	Adds httpx to dev dependencies.

Review details

• Files reviewed: 5/6 changed files
• Comments generated: 0
• Review effort level: Lite

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