feat: Proxy handler

[tusharpandey13]

This PR adds the handleProxy handler, a transparent and secure authenticating proxy to handle secure API calls originating from browser.

Changes:

• auth-client.ts:
• • added handleProxy, handlePreflight
• • added proxyMatcher in handler to enable future use-case support and to enable exhaustive testing of the proxy handler
• utils/proxy.ts: header filters, url transform, proxy matcher

References

• RFC 2616
• RFC 7231
• RFC 6454
• RFC 9449
• WHATWG Fetch Spec

Tests

proxy-handler.test.ts: Added tests for proxy handler covering:

• Basic Proxy Routing & Session Management
• HTTP Method Routing
• URL Path Matching & Transformation
• HTTP Headers Forwarding
• Request Body Handling
• Bearer Token Handling
• DPoP Token Handling
• Session Update After Token Refresh
• Error Scenarios
• Concurrent Request Handling
• CORS Handling

proxy.test.ts: Tests for header filters, url transform, proxy matcher

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.57321% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.50%. Comparing base (4a5834f) to head (529ed74).

Files with missing lines	Patch %	Lines
src/server/auth-client.ts	95.23%	6 Missing ⚠️
src/test/proxy-handler-test-helpers.ts	95.41%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2398      +/-   ##
==========================================
+ Coverage   87.90%   88.50%   +0.59%     
==========================================
  Files          37       39       +2     
  Lines        4127     4436     +309     
  Branches      818      878      +60     
==========================================
+ Hits         3628     3926     +298     
- Misses        496      507      +11     
  Partials        3        3              

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