feat: add Basic and Digest authentication for upstream connections

[myhomeiot]

MCP servers behind embedded Apache (or similar built-in HTTP frontends) often require Basic or Digest authentication. Bypassing that layer is usually impractical. Basic can already be forced with a static Authorization header, but Digest needs a proper challenge/response flow — and that is where most setups fall short.

Aside from a generic reverse proxy — which is not always convenient — I could not find an MCP proxy with Digest support for upstream connections. This change would make mcp-proxy the first of its kind in that niche.

mcp-proxy already forwards httpx.Auth into the Streamable HTTP and SSE clients, with OAuth2 client credentials wired the same way. Opening Basic and Digest through the CLI is a small, natural extension — reuse the existing HTTPX capability without changing the MCP transport.

• --username / --password → httpx.BasicAuth
• --username / --password / --digest → httpx.DigestAuth
• full OAuth2 (--client-id + --client-secret + --token-url) still takes priority

Uses stock httpx.DigestAuth — no custom Digest implementation.

Verified against a real Streamable HTTP MCP endpoint with both Basic and Digest, including a full handshake rather than a single HTTP request.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mcp_proxy/__main__.py	25.00%	12 Missing ⚠️

❌ Your patch check has failed because the patch coverage (25.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
- Coverage   89.37%   88.67%   -0.71%     
==========================================
  Files          12       12              
  Lines        1318     1333      +15     
  Branches       72       75       +3     
==========================================
+ Hits         1178     1182       +4     
- Misses        121      132      +11     
  Partials       19       19              

Files with missing lines	Coverage Δ
src/mcp_proxy/__main__.py	44.44% <25.00%> (-1.82%)	⬇️

... and 1 file with indirect coverage changes

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