Migrate YouTube transcript clients to v1.x with proxy support - #1731

Outcome

Routes YouTube transcript and yt-dlp traffic through the v1.x APIs and centralized outbound proxy handling, with credentials redacted from errors.

Scope

• Included:
  • Migrated transcript clients to instance-based fetch() / list() APIs.
  • Applied centralized proxy configuration to transcript and yt-dlp calls.
  • Hardened malformed proxy URL handling.
  • Redacted proxy credentials in transcript-related logs.
  • Added focused regression coverage.
• Explicitly excluded:
  • Changes to unrelated YouTube integrations or deployment configuration.

Risk

• Risk level: low
• Failure mode: Invalid proxy configuration falls back to direct connections; transcript clients retain existing fallback behavior.
• Rollback: Revert the migration commit.

Verification

• Focused regression coverage added
• Python compilation and whitespace checks pass
• Secret scan passes
• CodeQL reports zero alerts
• Required CI
• Review threads resolved

Production evidence

Not applicable; this change affects shared client behavior and requires normal CI/deployment validation.

Agent handoff

• One canonical issue is linked
• No competing PR implements the same issue
• Acceptance criteria are satisfied
• Static validation and security checks pass on the current head
• Human review requested for merge approval

• Fixes Migrate YouTube transcript client to v1.x with proxy support #1087