test: expand frontend API integration coverage

[MrDongsls]

Propose

Close #46

• Expand frontend API integration coverage for decision fixtures.
• Add tests for invalid routes and backend URL prefixes with query parameters.
• Run the focused frontend test suite in CI and document the command.

Test Plan

System1-Omni Version / Commit: 11f73ea

Test Result

• cargo fmt --all --check
• cargo clippy --workspace --locked --all-targets -- -D warnings
• cargo test -p omni-jev --test frontend --locked — 12 passed
• cargo test --workspace --locked
• cargo build --workspace --release --locked

GPU kernel tests were ignored because this environment has no GPU/CUDA runtime.

Self-review

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• I have reviewed the full diff and addressed the issues I found.
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[hsliuustc0106]

Reviewed and verified locally at 11f73ea.

• cargo fmt --all --check — clean
• cargo clippy -p omni-jev --all-targets --locked -- -D warnings — clean
• cargo test -p omni-jev --test frontend --locked — 12 passed, matching the count reported in the PR description

The three new tests assert real behavior at the transport boundary rather than tautologies: the decision fixtures lock in byte-for-byte preservation in both directions, the invalid-route cases verify the router's 405/404 semantics and that rejected requests never reach the backend (the mock worker records everything through a fallback handler, so that assertion is meaningful), and the prefix + query test covers query-string forwarding under a /worker backend prefix, complementing the existing health-path test. The scope matches #46's proposed first contribution, so closing it on merge is reasonable.

Two non-blocking nits, no changes required:

1. The new "Frontend API tests" CI step runs tests that cargo test --workspace already runs one step later, so it duplicates coverage for the sake of readable failures. The issue explicitly asked for this, so keeping it is fine — just noting the trade-off.
2. round_trips_decision_fixtures indexes seen[0] without a seen.len() == 1 check, unlike its sibling tests. Purely stylistic; a failure would still surface.

One housekeeping note: CI currently shows "no checks reported" because this PR touches .github/workflows/ci.yml from a fork — the workflow runs are held until a maintainer approves them in the Actions tab.