fix(strategy_v2): accept positional key/value in DiscoveryContext.set_metadata

[SAY-5]

Summary

Generated strategies keep calling context.set_metadata("direction_mode", "long_only") positionally, and DiscoveryContext.set_metadata only takes keyword arguments, so POST /api/strategies/generate fails at compile time and the auto-repair round trips into the same TypeError. Accept a (key, value) pair or a mapping in addition to keyword arguments, and spell out the keyword form in the generation prompt so the model is less likely to produce the positional call in the first place.

Fixes #208

Changes

• strategy_v2/contract.py: set_metadata accepts (key, value) and a mapping as well as keyword arguments
• ai_generation_contracts.py: show the keyword-argument form of set_metadata in the prompt
• regression test in test_strategy_v2_contract.py

Test plan

• Tested locally with docker compose up -d --build
• Backend logs show no errors
• Relevant pytest tests pass (tests/test_strategy_v2_contract.py, tests/test_strategy_v2_runtime.py, tests/test_ai_generation_prompt_contracts.py)

API documentation (if routes/schemas changed)

• Regenerated docs/api/openapi.yaml (cd backend_api_python && python scripts/export_openapi.py)
• Updated docs/agent/agent-openapi.json if /api/agent/v1 routes changed
• Breaking API changes called out below (oasdiff will fail CI otherwise)

Screenshots (if UI change)