Deepen HTTP request validation: method-aware policy + dedicated validator module

[XuebinMa]

Summary

Split from #3. The write_file and read_file validator parity is done (see crates/agent-guard-validators/src/path.rs and crates/agent-guard-core/src/file_paths.rs), but HTTP validation depth remains shallow: there is no dedicated http.rs validator module today, and policy matching is primarily URL-centric.

Scope

• introduce a dedicated http_request validator module that extracts method semantics before policy/runtime execution
• allow policy rules to match on method, body content type, and (optionally) body presence beyond URL prefix/regex
• tests for method-sensitive policy paths and obvious URL-encoding bypass cases

Out of Scope

• new policy DSL features beyond what existing rule types need
• ingress/inbound HTTP gating (this issue is outbound-mutation only)

Success Criteria

• crates/agent-guard-validators/src/http.rs exists and is wired into the runtime path
• README can honestly say HTTP policy is method-aware, not just URL-centric
• tests cover method-sensitive policy decisions and obvious bypass attempts