tests: clean up integration fixtures with DeleteIntegration

[winhowes]

Motivation

• Several integration tests were leaking state into the global integration registry by only stopping limiter goroutines, causing intermittent failures like "integration ... already exists" when tests ran repeatedly or in different orders.
• Use the public cleanup function to ensure both the registry entry and associated limiters are removed to avoid cross-test interference.

Description

• Replace direct calls to i.inLimiter.Stop() / i.outLimiter.Stop() with DeleteIntegration(i.Name) in tests.
• Applied the change in TestIntegrationRateLimitWindow, TestIntegrationTransportSettings, and TestIntegrationPluginTransport in app/integration_test.go.
• This centralizes cleanup so the integration entry is removed from the global map and its limiters are stopped consistently.

Testing

• Ran go test ./app -run 'TestIntegrationRateLimitWindow|TestIntegrationPluginTransport' -count=1 and both tests passed.
• Ran go test ./app -count=1 and the package tests passed.
• Ran go test ./... -count=1 and the full test suite passed.

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Codex Task

[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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