[Superseded by #97] Add stateless backend-scoped task handles

[lucarlig]

Summary

• add the versioned cfth1 stateless task-handle codec for modern MCP Tasks
• protect upstream task routes with misuse-resistant AES-256-GCM-SIV and replica-safe random nonces
• bind handles to the trusted authorization-context ID, virtual host, configuration revision, immutable backend ID, and backend generation
• require current effective-configuration validation on decode and return indistinguishable invalid-task errors without exposing upstream IDs
• document that this codec is an architecture prerequisite; task creation and lifecycle proxy handlers remain follow-up work
• update h2 to patched 0.4.16 after the fresh CI advisory database flagged 0.4.15

Why this is needed

Modern MCP Tasks return identifiers that clients reuse later with tasks/get, tasks/update, and tasks/cancel. Those identifiers are backend-specific and may behave like bearer tokens. Returning them directly would expose upstream details and would not safely preserve which backend owns a task.

The future dataplane is horizontally scaled and stateless. A process-local task map would require session affinity, while a Redis task map would add hot-path state, expiry, and cleanup. An encrypted routing handle carries the minimum routing context and can be decoded by any replica sharing the key.

Binding the handle to the trusted authorization context, virtual host, configuration revision, backend ID, and backend generation prevents cross-tenant or cross-policy replay and fails closed after backend removal or reassignment. It also keeps authorization current: every lifecycle request must still validate the JWT, load effective configuration, enforce scope/RBAC policy, and verify the backend generation before making an upstream call.

This PR provides that security and routing primitive only. It does not expose Tasks methods or make the codec current handler behavior; those handlers remain follow-up work.

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace — 231 passed
• cargo deny fetch && cargo deny check advisories licenses
• cargo build --locked --workspace
• cargo shear --check-test-targets --deny-warnings --locked
• mdbook build _context/wiki
• mdbook test _context/wiki

Closes IBM/mcp-context-forge#6185

[lucarlig]

Stack migration note: GitHub recorded this PR as merged into user/luca/stabilize-gateway-test-ports, not into main, while the branch ancestry was being corrected. The task-handle review continues in #97, with #95 above it in native GitHub stack #98.