feat: Add Gemini pricing tier config option

[pschoudhary-dot]

This commit introduces a new configuration option use_standard_pricing_only that allows users to force the use of the standard pricing tier for Gemini models, even for large contexts. in relevance of this issue 592

• Modified models.ts and provider.ts to handle tiered pricing.
• Added use_standard_pricing_only to config.ts and updated config.schema.json.
• Added tests for the pricing tier logic in provider.test.ts.
• Updated documentation in config.mdx.