fix: enforce MCP parameter header requirements

[lucarlig]

Summary

• allow tool_schemas to be omitted and validate parameter headers only when the control plane publishes the called tool's input schema
• validate the MCP 2026-07-28 x-mcp-header contract: token syntax, case-insensitive uniqueness, primitive types, properties-only nested paths, exact value extraction, null/absence behavior, safe integers, numeric comparison, Base64 decoding, unsafe plain values, and repeated headers
• forward unrecognized Mcp-Param-* headers unchanged when no matching annotation is published
• update focused integration/unit coverage, the generated user-config schema, and hot-path wiki documentation

Behavior

Published annotation	Argument	Header	Result
no	any	absent or present	no local validation; forward present headers
yes	absent or null	absent	allow
yes	absent or null	present	HeaderMismatch (-32020)
yes	present and non-null	absent	HeaderMismatch (-32020)
yes	present and non-null	present	decode and validate against the exact argument path

Malformed annotations invalidate validation for the tool call. Malformed, conflicting, or mismatched recognized header values fail closed before the backend is called.

RMCP dependency

The dataplane returns the required HeaderMismatch JSON-RPC error. The required HTTP 400 mapping is tracked by upstream RMCP PR modelcontextprotocol/rust-sdk#1226; this PR intentionally does not duplicate or vendor that transport fix.

Validation

• rebased onto main at 29a2a81
• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace (229 passed, 11 skipped)
• cargo build --locked --workspace
• cargo deny check advisories licenses (passed; existing transitive yanked chacha20 0.10.1 warning through rmcp)
• cargo shear --check-test-targets --deny-warnings --locked
• mdbook build _context/wiki
• mdbook test _context/wiki