fix(http): refine Proxy-Authenticate basic auth workflow

[tianhuyang]

Summary

This PR extends the original HTTP proxy auth header formatting fix with follow-up reliability and observability improvements in protocol/http/handshake.go.

What Changed

• Kept the Proxy-Authenticate header formatting fix:
  • Basic realm="sing-box", charset="UTF-8"
• Added one-time retry for missing Proxy-Authorization when retry is safe.
• Added configurable retry timeout through HTTPServerOptions.ProxyAuthRetryTimeout (default 5s).
• Migrated handshake logging to repository logger abstraction:
  • Added HTTPServerOptions.Logger (logger.ContextLogger) with logger.NOP() fallback.
  • Logged request protocol and response protocol+status.
  • Logged request/response headers in deterministic key order.
• Redacted sensitive header values in logs:
  • Authorization, Proxy-Authorization, Cookie, Set-Cookie.
• Preserved retry safety constraints:
  • no retry on upgrade,
  • no retry for requests with body/transfer-encoding,
  • keep-alive aware behavior across HTTP/1.0 and HTTP/1.1 semantics.

Tests

Added/updated tests in protocol/http/handshake_test.go, including:

• retry success/once/timeout behavior,
• HTTP/1.0 keep-alive and default-close cases,
• no-retry on upgrade and request body,
• logger emission checks,
• sensitive-header redaction checks.

Validation

• CGO_ENABLED=0 go test ./protocol/http
• CGO_ENABLED=0 go test ./...

Notes

This PR now covers the full set of HTTP proxy auth handshake updates currently on branch fix/http-proxy-auth-header-comma.

[tianhuyang]

@nekohasekai @wwqgtxx could you please take a look at this small header-format fix when convenient? Thank you.

[tianhuyang]

Not needed