Share HTTP client construction between gRPC calls and reflection

[ryanfowler]

Summary

• Extracted reqwest client setup into a shared builder used by both normal HTTP execution and gRPC reflection/discovery
• Restored parity for reflection with request-path options: --min-tls, --max-tls, proxy handling, custom DNS, sessions, and timing connector support
• Kept reflection on HTTP/2 prior knowledge while preserving request-mode-specific protocol selection for normal traffic
• Added integration coverage for reflection TLS bounds, proxy rejection/allowance, and --dns-server

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1