Gemini models update.

[npatsakula]

Fixes #75

Syncs the Model enum with the Gemini API model lineup.

Changes:

• Default model is now Model::Gemini37Flash (gemini-3.7-flash) — gemini-2.5-flash is no
  longer available to newly created API keys
• Added current models: Gemini 3.6/3.5/3.1 Flash family, gemini-3.1-pro-preview,
  Nano Banana 2 image models, TTS models, gemini-omni-flash, gemini-2.5-computer-use-preview,
  and embeddings (gemini-embedding-2, gemini-embedding-001)
• Fixed stale mappings: Gemini3ProImage → GA gemini-3-pro-image; Gemini::pro() now uses
  Model::Gemini31Pro
• Deprecated: Model::Gemini3Pro and Model::TextEmbedding004 (both shut down server-side),
  with migration notes
• Updated README, crate docs, doc examples, and examples off shut-down models