feat: add Azure passthrough support

[Pratham-Mishra04]

Summary

Adds Azure passthrough functionality to enable direct API forwarding to Azure endpoints without transformation, and improves error messaging by removing redundant model information from key selection errors.

Changes

• Implemented Passthrough and PassthroughStream methods for Azure provider to forward requests directly to Azure's API
• Added Azure passthrough router with /azure_passthrough endpoint integration
• Simplified error messages in key selection and routing rule validation by removing model-specific details
• Added proper authentication, header handling, and streaming support for Azure passthrough requests

Type of change

• Feature
• Bug fix

Affected areas

• Core (Go)
• Transports (HTTP)
• Providers/Integrations

How to test

Test the Azure passthrough functionality:

# Test core functionality
go test ./core/providers/azure/...
go test ./core/...

# Test HTTP transport integration
go test ./transports/bifrost-http/...

# Test passthrough endpoint
curl -X POST http://localhost:8080/azure_passthrough/your-endpoint \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer your-token" \
  -d '{"your": "request"}'

Verify error message improvements by triggering key selection errors and routing rule conflicts.

Screenshots/Recordings

N/A

Breaking changes

• Yes
• No

Related issues

N/A

Security considerations

• Azure passthrough properly handles authentication headers using existing getAzureAuthHeaders method
• Safe header filtering prevents sensitive headers from being exposed
• Request validation ensures proper Azure key configuration before processing

Checklist

• I read docs/contributing/README.md and followed the guidelines
• I added/updated tests where appropriate
• I updated documentation where needed
• I verified builds succeed (Go and UI)
• I verified the CI pipeline passes locally if applicable

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✨ Finishing Touches

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[Pratham-Mishra04]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: add Azure passthrough support #2135 👈 (View in Graphite)
• fix: apply MCP tool filtering headers to tools/list response when using bifrost as MCP gateway #2127 : 1 other dependent PR (#2151 )
• feat: add request-level extra headers support for MCP tool execution #2126
• refactor: replace string slices with WhiteList for allowlist fields #2125
• refactor: standardize empty array conventions for allowed models #2113
• refactor: standardize empty array conventions for VK Provider Config Allowed Keys #2006 : 1 other dependent PR (#2008 )
• feat: VK MCP config now works as an AllowList #1940
• feat: add MCP auto tool injection toggle #1933
• refactor: standardize empty array conventions for VK Provider & MCP Configs, and makes Provider Config weight optional for routing #1932
• v1.5.0

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