feat: Add response metadata fields

[aydinfatih]

Refs #168

Adds output fields from the API reference that are currently dropped while parsing.

Changes

Class	New property
GenerateContentResponse	responseId, modelStatus (ModelStatus)
Candidate	finishMessage, urlContextMetadata (UrlContextMetadata)
UsageMetadata	serviceTier (ServiceTier)

• New data classes: ModelStatus, UrlContextMetadata, UrlMetadata
• New enums: ModelStage, UrlRetrievalStatus, ServiceTier
• finishMessage explains why the model stopped, which helps with empty responses like the ones in Updating to gemini 2.5 flash I get a GenerateContentResponse error #168.

Backward compatibility

• New constructor parameters are nullable and appended at the end.
• Missing fields resolve to null.
• New enum values are parsed with tryFrom, like DocumentState, PromptFeedback and ModalityTokenCount, so values added to the API later resolve to null instead of failing the whole response.
• urlRetrievalMetadata is kept as is.

Tests

• All new fields parsed from a full response, including toArray() round trip
• Unknown enum values resolve to null
• Responses without the new fields still parse