Add WithDialer option for custom TCP dialer injection

[myleshorton]

Summary

• Adds a WithDialer(func(ctx context.Context, network, addr string) (net.Conn, error)) option to dnstt
• Threads the custom dialer through both dohDialer and dotDialer transport implementations via new utlsDialContextWithDialer and NewUTLSRoundTripperWithDialer functions
• Falls back to (&net.Dialer{}).DialContext when no custom dialer is provided (preserving existing behavior)
• Refactors utlsDialContext, NewUTLSRoundTripper, and makeRoundTripper to delegate to new dialer-aware variants

Test plan

• go build ./... compiles cleanly
• go vet ./... passes
• Existing tests pass (go test ./...)
• Verify DoH transport works end-to-end with default dialer
• Verify DoT transport works end-to-end with default dialer
• Verify custom dialer is called when injected via WithDialer

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds support for custom TCP dialer injection to the dnstt package, allowing callers to control how TCP connections are established for DNS-over-HTTPS (DoH) and DNS-over-TLS (DoT) transports. This is useful for scenarios like proxying, custom DNS resolution, or network-level connection control.

Changes:

• Adds WithDialer option to configure a custom TCP dialer function
• Threads the custom dialer through both DoH and DoT transport implementations
• Refactors uTLS helper functions to support optional custom dialers while maintaining backward compatibility

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dnstt.go	Adds WithDialer option, injects custom dialer into transport structs via type switch, adds dialContext field to dohDialer and dotDialer structs
utls.go	Adds utlsDialContextWithDialer and NewUTLSRoundTripperWithDialer functions, refactors existing functions to delegate to new dialer-aware variants, adds dialContext field to utlsRoundTripper

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[Copilot]

The dialer injection uses a type switch that silently does nothing if the transport is not a recognized type. If a new transport type is added in the future and the developer forgets to update this switch statement, the custom dialer will be silently ignored.

Consider one of these alternatives:

1. Add a default case that logs a warning or returns an error
2. Add a method to the transport interface like setDialer(func(...) (net.Conn, error)) to make this more explicit and type-safe
3. Pass the dialer directly when creating the transport in WithDoH and WithDoT options (though this would require refactoring)

Suggested change

	t.dialContext = dial
	t.dialContext = dial
	default:
	slog.Warn("unable to inject custom dialer into transport; unrecognized transport type", "transportType", fmt.Sprintf("%T", t))

[Copilot]

The new WithDialer option lacks test coverage. Consider adding tests to verify:

1. The custom dialer is actually called when making connections through DoH
2. The custom dialer is actually called when making connections through DoT
3. The behavior when no custom dialer is provided (default fallback works correctly)
4. Error handling when the custom dialer returns an error

This is especially important since the dialer injection happens via type assertion and could fail silently if the code changes in the future.

[Copilot]

The parameter name dialCtx is inconsistent with the naming used elsewhere in the codebase. In utlsDialContextWithDialer and NewUTLSRoundTripperWithDialer, the parameter is named dialContext, but here it's abbreviated to dialCtx.

Consider renaming to dialContext for consistency.