feat: add 54 OpenAI-compatible providers with namespace-based model organization

[surafelfikru]

This PR adds 54 new OpenAI-compatible providers to the SDK, bringing comprehensive provider coverage. It consolidates the provider code generation system and fixes critical issues with model naming and struct `duplication.

Major Changes

New Providers (54 total):

• Core providers: Mistral, Chutes, CloudflareAiGateway, FireworksAi
• Regional providers: AlibabaCn, MoonshotaiCn, SiliconflowCn
• GitHub integration: GithubCopilot, GithubModels
• Specialized tiers: ZaiCodingPlan, ZhipuaiCodingPlan
• Plus 42 additional OpenAI-compatible providers

Public API Additions:

• New provider exports in src/providers/mod.rs (all 54 providers)
• Individual feature flags for each provider (e.g., chutes, fireworks-ai, github-copilot)
• Updated full feature flag to include all new providers

Code Generation Improvements:

• Consolidated provider-codegen.py as single source of truth
• Enhanced duplicate struct name detection and resolution
• Improved Pro/ prefix model handling (differentiates variants)
• Better namespace organization for slash-based model IDs

Bug Fixes

1. Duplicate Struct Names: Fixed conflicting implementations where models with version variants (e.g., claude-3-5-haiku vs claude-3.5-haiku) generated identical struct names. Now properly differentiated (e.g.,
   Claude35Haiku vs Claude3Dot5Haiku).

2. Pro Prefix Models: Models with Pro/ prefix now generate distinct struct names with Pro prefix to avoid conflicts with non-Pro variants (e.g., ProDeepseekR1 vs DeepseekR1).

3. Special Character Handling: Proper handling of:

   • Module names starting with digits (302ai → ai_302)
   • Hyphenated provider names (github-copilot → github_copilot)
   • PascalCase struct generation consistency

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have read the Contributing Guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (CHANGELOG.md)
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules