feat(ai): expose Gemini rate-limit retry metadata

[vitorhugo-dotnet]

Implements the backend portion of #87.

What changed

• maps Gemini 429 RESOURCE_EXHAUSTED / quota failures to a structured RATE_LIMITED SSE error
• extracts the provider-provided retryAfterSeconds when Gemini includes a retry delay
• does not invent a retry delay when the provider omits it
• keeps non-rate-limit failures as PROVIDER_UNAVAILABLE
• isolates Gemini-specific error inspection in AssistantProviderErrorMapper
• keeps the controller responsible only for emitting the mapped SSE payload

Verification

• TDD RED confirmed before implementation
• mvn -B -ntp clean verify ✅
• Qodana ✅

Frontend companion: vitorhugo-dotnet/React-JobApplyTracker#115

[github-actions]

Qodana for JVM

3 new problems were found

Inspection name	Severity	Problems
Nullability and data flow problems	🔶 Warning	1
Mismatched query and update of 'StringBuilder'	🔶 Warning	1
Simplifiable conditional expression	🔶 Warning	1

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