feat: add new AI providers and update existing ones

[cpunion]

• Add new providers: Gemini, Groq, and SambaNova
• Update OpenAI and Anthropic client implementations
• Add .gitignore file
• Update README and core client functionality

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 20 lines in your changes missing coverage. Please review.

Project coverage is 62.46%. Comparing base (9e43822) to head (02b9687).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
providers/providers.go	0.00%	10 Missing ⚠️
providers/anthropic/anthropic_provider.go	66.66%	1 Missing and 1 partial ⚠️
providers/gemini/gemini_provider.go	75.00%	1 Missing and 1 partial ⚠️
providers/groq/groq_provider.go	75.00%	1 Missing and 1 partial ⚠️
providers/openai/openai_provider.go	66.66%	1 Missing and 1 partial ⚠️
providers/sambanova/sambanova_provider.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   61.98%   62.46%   +0.47%     
==========================================
  Files           8       11       +3     
  Lines         342      389      +47     
==========================================
+ Hits          212      243      +31     
- Misses        117      130      +13     
- Partials       13       16       +3     

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