fix(fallback): preserve provider-specific headers when activating fallback

[SeeYangZhi]

Summary

Fixes #6075

When _try_activate_fallback() swaps to a provider that requires provider-specific HTTP headers (e.g. kimi-coding requires User-Agent: KimiCLI/1.x), the returned client from resolve_provider_client() correctly has the headers baked in. However, _client_kwargs was saved with only api_key and base_url, dropping default_headers.

Every subsequent API call rebuilds the client from _client_kwargs via _create_request_openai_client(), producing a bare OpenAI client without the required headers — causing 403s from Kimi Coding and potentially missing auth headers for GitHub Copilot.

Changes

# In _try_activate_fallback(), after self.client = fb_client:
fb_headers = getattr(fb_client, "_custom_headers", None)
if not fb_headers:
    fb_headers = getattr(fb_client, "default_headers", None)
self._client_kwargs = {
    "api_key": fb_client.api_key,
    "base_url": fb_base_url,
    **({"default_headers": dict(fb_headers)} if fb_headers else {}),
}

The OpenAI SDK stores default_headers passed to the constructor in client._custom_headers, so this correctly round-trips the headers through any client rebuild.

Verification

• Synthetic round-trip: OpenAI client created with KimiCLI headers → read from _custom_headers → passed to new client → headers preserved
• E2E: AIAgent with bogus primary key + kimi-coding fallback. Primary 401s → fallback activates → 3 consecutive calls all succeed with KimiCLI/1.3 in _client_kwargs. Without the fix, turns 2+ fail with 403.

Related

• [Bug]: Sync auxiliary vision calls fail with 403 on kimi-coding custom endpoint due to missing User-Agent header #5796 / fix(auxiliary_client): inject KimiCLI User-Agent for custom endpoint sync clients #5797 — Same class of bug (missing KimiCLI User-Agent) in the auxiliary client custom endpoint path
• fix(auth): kimi-coding pool base_url seeding + PKCE endpoint fallback #5669 — Fixes kimi-coding credential pool base_url seeding (different bug, same provider)

[teknium1]

Merged via salvage PR #6754. All three commits cherry-picked onto current main with your authorship preserved in git log. Thank you @SeeYangZhi!