fix: gRPC detection can now connect to real (h2c) gRPC servers

[GovindarajanL]

Fixes #72

Summary

GrpcEndpointDetectionTestCase probed for gRPC services over the shared HttpClient, which wraps a standard OkHttp client. gRPC's wire protocol is HTTP/2-only, and OkHttp never attempts HTTP/2 over a plain http:// URL by default (there's no ALPN negotiation without TLS) — it silently falls back to HTTP/1.1, which a gRPC server cannot understand at all. The probe request fails at the transport layer before there's ever a response to inspect.

Reproduction

Built and ran gRPC Goat labs 001 (server reflection enabled) and 002 (plaintext gRPC), both confirmed live via docker logs. Scanning both produced 0 findings on both, including lab001 — the "reflection enabled" scenario this test case is specifically meant to catch.

Confirmed the transport-level cause by hand:

$ curl -s -o /dev/null -w "%{http_code}" http://127.0.0.1:8001/
000   # plain HTTP/1.1 — connection never completes

$ curl -s -o /dev/null -w "%{http_code}" --http2-prior-knowledge http://127.0.0.1:8001/
415   # real HTTP/2 connection succeeds — server is genuinely reachable

Fix

Adds HttpClient.postH2c(), which routes cleartext (http://) targets through a dedicated client configured for Protocol.H2_PRIOR_KNOWLEDGE. https:// targets are unaffected and continue through the standard client, which already negotiates HTTP/2 via ALPN when the server supports it — no change to any other test case's behavior.

Verification

• Re-ran against both gRPC Goat labs after the fix: 2/2 detection, up from 0/2. Both now report the gRPC endpoint detection (INFO) and "gRPC Server Reflection Enabled" (MEDIUM).
• Existing unit tests updated to mock postH2c instead of postWithStatus; all still pass.
• Full suite passes (241 tests).

Test plan

• mvn test passes
• Re-scanned gRPC Goat lab001 and lab002 with the fixed jar — both correctly detected, including reflection