fix(desktop): isolate relay admission gate tests

[BrianInAz]

Summary

• add a shared test-only isolation boundary for the process-wide relay admission gate
• reset the gate before each participating test and again when the guard drops
• make the cross-module HTTP 429 error-message test participate in the same boundary

Root cause

oversized_hint_is_capped_in_relay_error_message_string exercised a real 429 response and armed the process-wide admission gate for the capped 300-second window. Because that test lived outside relay_admission, it did not acquire the module's shared test lock or clean up the gate. Parallel gate tests could then observe 300.001 seconds instead of their expected 5-second window.

All changes are compiled only for tests. Production admission behavior, hint clamping, and relay semantics are unchanged.

TDD evidence

• Red: the targeted contract failed before the shared isolate_test_gate boundary existed
• Green: isolation cleanup regression and the offending 429 test pass
• relay library subset passed 110 tests with 5 expected ignores on five consecutive runs
• Desktop Tauri formatting and Clippy passed
• Desktop Tauri tests passed 2,115 tests with 15 expected ignores, plus 3 diagnostic tests
• full just ci passed, including 1,098 mobile tests with 1 expected skip
• git diff --check passed

Beads: ios-buzz-59e.11

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