chore(router): simplify unknown-model error message construction

[stuxf]

Changes

The unknown-model error message in Router is already fully built by the f-string interpolation, so the trailing .format() call on the resulting string was redundant. This removes it and adds a regression test that the message renders the model name verbatim.

Type

🧹 Refactoring

[greptile-apps]

Greptile Summary

This PR simplifies Router unknown-model error message construction. The main changes are:

• Removed redundant .format() calls after f-string interpolation in litellm/router.py
• Added a focused regression test for model names containing Python format-field syntax
• Confirmed the unknown-model message preserves the caller-supplied model name literally

Confidence Score: 5/5

The change is narrowly scoped to redundant string formatting removal and is covered by a focused regression test.

The implementation preserves the existing constructed error message while eliminating the extra formatting step, and the added test exercises the relevant edge case for literal model names.

T-Rex Logs

What T-Rex did

• The before-revision repro for the router-unknown-model message finished with EXIT_CODE: 0.
• The after-revision repro finished with EXIT_CODE: 0 and produced an improved head error string that includes You passed in model=ghost with literal_model_contained: True and padding_run_detected flags: False.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "chore(router): simplify unknown-model er..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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