fix(proxy): skip pricing lookup for passthrough model labels

[Hughhhhcoder]

Description

Internal passthrough:<endpoint> model labels (used by passthrough handlers such as Anthropic /v1/messages/count_tokens) are not billable model calls. CostTracker passed them into LiteLLM for pricing lookup, which rejects them with LLM Provider NOT provided and emits a spurious pricing WARNING per process — plus a misleading passthrough:* row on the dashboard.

Closes #2578

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

• Added _is_passthrough_model() to detect internal passthrough: labels.
• Skip pricing lookup for those labels before touching LiteLLM, so no spurious WARNING is emitted.

Testing

• Unit tests pass (pytest)
• New tests added for new functionality

Test Output

$ pytest tests/test_cost_passthrough_pricing.py -q
3 passed

Real Behavior Proof

• Environment: macOS, Python 3.11
• Exact command / steps: pytest tests/test_cost_passthrough_pricing.py -q
• Observed result: 3 passed — passthrough labels return None with no WARNING; non-passthrough models still priced.
• Not tested: full headroom test suite (only the new/changed cost tests).

Runtime Rollout Safety

• Rollout-managed feature(s): N/A
• Minimum rollout channel: N/A
• Stable/default behavior changed: No — passthrough labels already failed pricing; this only removes the spurious WARNING.
• Kill switch / disable path: N/A (pure lookup skip, no new state)
• Unsafe override required: No
• Qualification impact: None
• Rollback path: Revert this commit.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes
• I did not edit CHANGELOG.md

Additional Notes

• Only the new/changed cost tests were run locally; the change is localized to the pricing lookup path and does not touch core proxy routing.

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.