test: add full-binary integration suite - #88

Summary

• build the proxy and a controllable stdio upstream once per integration suite
• exercise real stdio and HTTP subprocesses, including policy, rate limits, redaction, terminal audit outcomes, MCP 2026 metadata, static and OIDC authentication, incoming TLS, and JSONL/SQLite verification
• keep the suite opt-in with the integration build tag and run it in CI

Context

Closes #50

This PR is stacked on the temporary v1.2 integration branch so it can cover every preceding security workstream. It can be retargeted to main after the dependency PRs merge.

Test plan

• GOCACHE=/tmp/mcp-audit-go-cache go test -race ./...
• GOCACHE=/tmp/mcp-audit-go-cache go test -tags=integration ./tests/integration/...
• GOCACHE=/tmp/mcp-audit-go-cache go build ./...
• GOCACHE=/tmp/mcp-audit-go-cache go vet ./...

The integration suite completes in about six seconds locally.

Stability impact

None. This adds opt-in tests and a CI step without changing runtime contracts.