fix: honor explicit model provider routing

[Sushanth012]

Summary

• make explicit provider settings take precedence over model-prefix provider inference
• thread modelProvider through agent LLM calls, streaming fallback, memory flush, compaction, and context-window selection
• add provider resolution tests for explicit override, prefix fallback, and unknown override fallback

Testing

• bun test src/providers.test.ts
• bun test src/providers.test.ts src/controllers/agent-runner.test.ts
• bun run typecheck
• bun test

Closes #272