fix: forward SEP-2243 parameter headers

[lucarlig]

Summary

• forward client-supplied Mcp-Param-* headers unchanged on upstream tools/call
• keep those headers protected from backend pass-through, add, and remove configuration
• leave plugin payload handling unchanged; argument rewrites do not rewrite the copied headers
• let RMCP regenerate Mcp-Method, the routed Mcp-Name, and MCP-Protocol-Version

The dataplane does not fetch tool schemas, call tools/list, decode, validate, or reconstruct parameter headers. The upstream MCP server owns validation.

Stacked on #107. No control-plane change, RMCP update, or new runtime dependency is required.

Related: IBM/mcp-context-forge#6256.

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace (223 passed, 3 skipped)
• workspace build, dependency-policy, shear, and wiki checks
• MCP_CONFORMANCE_COLOR=never make conformance (four shell-adapter generation check IDs baselined; no unexpected failures)
• focused tests cover opaque mismatched-value forwarding, backend-config protection, and plugin payload rewrites