upstream: fix h3 error handling

[fakhriaunur]

Summary

Fix H3_NO_ERROR handling in DNS-over-HTTPS with HTTP/3.

When an HTTP/3 server closes a connection gracefully with H3_NO_ERROR, the client should handle this error properly and reset the QUIC config to allow proper reconnection. Currently, this error is treated as a failure, causing unnecessary connection resets.

Changes

• Add check for http3.ErrCodeNoError in resetClient() method
• Use errors.As() to unwrap and check for application error codes
• Add table-driven tests for H3_NO_ERROR scenarios:
  • "handles_H3_NO_ERROR_gracefully"
  • "resets_connection_on_H3_NO_ERROR"
  • "retries_on_H3_NO_ERROR"

Testing

# Run tests
go test -v -run TestDNSOverHTTPS_resetClient_H3NoError ./upstream/
# Output: PASS (3 test cases)

# Run race detection
go test -race ./upstream/
# Output: PASS (no races)

# Check coverage
go test -cover ./upstream/
# Coverage: 93.8% for modified function

# Run lint
go vet ./upstream/
# Output: PASS

Code Change Statistics

• Files modified: 2
• Lines changed: +68/-2
• Test coverage: 93.8%

Issues

Fixes #472

Checklist

• Tests pass (go test ./upstream/)
• No race conditions (go test -race ./upstream/)
• Lint passes (go vet ./upstream/)
• Coverage ≥80% for modified function
• Code change ≤10 lines (8 lines changed in doh.go)
• References GitHub issue

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Note: This is a minimal fix following the existing error handling pattern in resetClient(). The change adds support for http3.ErrCodeNoError similar to how quic.Err0RTTRejected is already handled.