Add JSON Mode Support for Gemini Adapter

[nshkrdotcom]

Feature Request: Add support for :json mode in the Gemini adapter to enable generating JSON responses without a strict schema.

Description:

The Gemini adapter currently supports :json_schema mode for structured JSON output with a defined schema. This feature enhancement adds support for :json mode, which allows the Gemini API to generate JSON responses by setting response_mime_type: "application/json" without a response_schema. This is useful for scenarios where flexible JSON output is needed without enforcing a specific structure.

Proposed Changes:

• Update lib/instructor/adapters/gemini.ex to include :json in @supported_modes.
• Modify the chat_completion function to handle :json mode by setting response_mime_type: "application/json" and removing the response_format field to avoid API errors.
• Add response parsing for :json mode in parse_response_for_mode/2 to decode JSON text.
• Add streaming support for :json mode in parse_stream_chunk_for_mode/2 to handle JSON chunks.
• Update tests in test/instructor_test.exs to include :json mode for the Gemini adapter (model: gemini-2.5-flash-preview-04-17).

Notes:

• The pull request includes preparatory changes for :tools mode (e.g., mode handling in chat_completion), but these are not fully implemented or tested yet and will be addressed in a future issue.
• The implementation addresses a previous issue where response_format caused 400 errors by explicitly removing it for :json mode.
• One streaming test (streams arrays one at a time) is failing due to invalid JSON output from the Gemini model. This may be addressed in a follow-up issue to improve prompt instructions or chunk cleaning.

Acceptance Criteria:

• All tests for :json mode (except the known streaming test) pass with mix test --only adapter:gemini test/instructor_test.exs.
• The adapter correctly generates and parses JSON responses in non-streaming scenarios.
• Streaming responses are partially supported, with the known issue documented for future resolution.

Related Issues:

None yet, but a follow-up issue will be created for the streaming test failure and :tools mode implementation.