Handle rate limits more gracefully

[AlCalzone]

ApiError:

{
  "error": {
    "code": 429,
    "message": "You exceeded your current quota, please check your plan and billing details. For more information on this error, head to: https://ai.google.dev/gemini-api/docs/rate-limits.",
    "status": "RESOURCE_EXHAUSTED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.QuotaFailure",
        "violations": [
          {
            "quotaMetric": "generativelanguage.googleapis.com/generate_content_free_tier_input_token_count",
            "quotaId": "GenerateContentInputTokensPerModelPerMinute-FreeTier",
            "quotaDimensions": {
              "location": "global",
              "model": "gemini-2.5-pro"
            },
            "quotaValue": "250000"
          }
        ]
      },
      {
        "@type": "type.googleapis.com/google.rpc.Help",
        "links": [
          {
            "description": "Learn more about Gemini API quotas",
            "url": "https://ai.google.dev/gemini-api/docs/rate-limits"
          }
        ]
      },
      {
        "@type": "type.googleapis.com/google.rpc.RetryInfo",
        "retryDelay": "15s"
      }
    ]
  }
}

This one should be fixable by waiting a bit.

[AlCalzone]

Along the same line we should handle other server errors too:

Chat message error: ApiError: {"error":{"message":"{\n  \"error\": {\n    \"code\": 503,\n    \"message\": \"The model is overloaded. Please try again later.\",\n    \"status\": \"UNAVAILABLE\"\n  }\n}\n","code":503,"status":""}}
    _ApiError errors.ts:25
    throwErrorIfNotOK _api_client.ts:760
    streamApiCall _api_client.ts:512
[gemini-client.ts:216:12](http://localhost:5173/log-analyzer/src/lib/ai/gemini-client.ts)
    sendChatMessage gemini-client.ts:216