fix(providers): strip provider prefix from model names for openai_com…

[Ajinkya-Ghuge]

…pat endpoints

Summary

Fixes an issue where OpenAI-compatible clients (like DeepSeek) fail due to strict provider/model naming conventions by dynamically stripping the provider prefix before sending the payload downstream.

Anti-slop triage

• Classification:
• Evidence:
• Non-destructive review result:

Verification

• Targeted tests/docs checks ran, or the gap is explicitly recorded.
• git diff --check passes.
• No live secrets, tokens, private logs, or unrelated generated churn are included.

Resolution gate

Closes #3229

• If this PR resolves an issue, the issue number and fix evidence are linked.
• If this PR should not merge, the rejection/defer rationale is evidence-backed and does not rely on vibes.
• I did not merge/close remote PRs or issues from an automation lane without owner approval.