fix(ai): retry every provider error

[bobleer]

Summary

• route every non-success provider HTTP response through the existing bounded retry loop, regardless of status code or structured error category
• retry every aggregated stream error until the existing attempt budget is exhausted
• remove model-loss, transient-category, permanent-keyword, and context-overflow retry admission checks while preserving cancellation, exponential backoff, rate-limit pacing, and Retry-After
• add regressions covering ordinary 400, context-overflow 400, and a formerly non-retryable SSE parsing error

Verification

• cargo test -p bitfun-ai-adapters (248 passed)
• cargo test -p bitfun-agent-stream (66 passed)
• pnpm run fmt:rs
• git diff --check

AI-assisted change: implemented with OpenAI Codex and verified with the focused repository test suites above.