feat(gemini): add Gemini 3 model constants and thinking_level support

[EinarasGar]

Summary

• Add model constants for Gemini 3 series (GEMINI_3_FLASH_PREVIEW, GEMINI_3_1_FLASH_LITE_PREVIEW)
• Add thinking_level: Option<String> field to ThinkingConfig for Gemini 3 models, which use thinkingLevel ("minimal", "low", "medium", "high") instead of Gemini 2.5's thinkingBudget
• Make thinking_budget optional (Option<u32>) so users can set either field depending on their model family

Context

Gemini 3 models (released March 2026) use a different parameter for controlling reasoning depth. Per the Gemini 3 Developer Guide, thinking_level and thinking_budget are mutually exclusive and cannot be set in the same request.

Both fields use skip_serializing_if = "Option::is_none" so existing Gemini 2.5 configurations that set thinking_budget via additional_params JSON continue to work unchanged.

Example usage

// Gemini 3
GenerationConfig {
    thinking_config: Some(ThinkingConfig {
        thinking_budget: None,
        thinking_level: Some("medium".to_string()),
        include_thoughts: Some(true),
    }),
    ..Default::default()
}

// Gemini 2.5 (unchanged)
GenerationConfig {
    thinking_config: Some(ThinkingConfig {
        thinking_budget: Some(2048),
        thinking_level: None,
        include_thoughts: Some(true),
    }),
    ..Default::default()
}

Test plan

• Tested with gemini-3-flash-preview — reasoning events emitted correctly
• Tested with gemini-2.5-flash — thinking budget still works as before
• cargo build passes