test(civisibility): cover JSON retry recovery

[darccio]

Summary

Adds the recovery-path unit test Tony suggested on merged PR #4917.

The existing test covered the retry-exhaustion path for repeated 2xx non-JSON responses and the default behavior when ExpectJSONResponse is false. This adds coverage for the main transient recovery scenario: the first 2xx response has a non-JSON Content-Type, then a retry returns a valid 2xx JSON response and succeeds.

Validation

• go test ./internal/civisibility/utils/net -run 'TestSendRequestExpectJSONResponse(RetriesOnUnknownFormat|RecoversAfterUnknownFormat)$' -count=1
• git diff --check

Note: full go test ./internal/civisibility/utils/net/... -count=1 failed locally in existing TestNewClient_DefaultValues with Expected serviceName to be set, got empty string, plus CI Visibility agentless API key logging. The targeted retry tests pass.

[datadog-official]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 62.87% (+4.01%)

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