switchyard_route: raise unit-test coverage to CI floor

[yehuditkerido]

Context

Parent: #2 · POC: #16

make coverage-check requires 90% lines / 80% regions workspace-wide. After #16, most of switchyard_route.rs is untested; config parse tests alone are not enough for the floor.

Goal

Bring praxis-experimental-filters (especially switchyard_route) up so make coverage-check passes without weakening the floor.

Proposed work

• Unit-test pure helpers first: judge URL parse, model rewrite, tier mapping, config edge cases, failure-mode handling.
• Add focused tests around route error paths (bad JSON, unsupported path, judge non-2xx) with mocks where needed.
• Prefer small testable extractions over a large HTTP integration harness if that gets coverage faster.

Notes

• Tests inside src/ count toward the PR size gate; consider a dedicated follow-up PR and/or skip/pr-conventions if needed.
• Demo scripts do not count for llvm-cov.

Acceptance criteria

• make coverage-check green on the branch.
• Meaningful assertions on success and failure paths (not only “code ran”).

[yehuditkerido]

/assign