[CF-DATAPLANE][CPEX] Resolve scoped bindings and typed extensions

[lucarlig]

Summary

• replace the singleton CPEX runtime with an atomically swapped catalog of immutable runtimes keyed by validated revision
• select the exact runtime revision from each authenticated principal and virtual-host binding, while pinning it across pre, streamed, and post lifecycle calls
• resolve and cache bounded, immutable dispatch plans for each ordered target binding before invoking CPEX entries
• reject unknown config fields, unsupported CPEX options, empty names/revisions/lists, duplicate names, stale revisions, and unknown or incompatible plugins
• pass None to CPEX when a post-only hook is the first lifecycle hook
• retain capability-gated typed extensions and canonical backend-local tool, prompt, and resource routing
• verify direct and namespaced password-resource redaction with the bundled secrets-detection plugin

Runtime contract

This intentionally replaces the unstable version 2 single-snapshot document. Version 3 is an atomic catalog:

{
  "version": 3,
  "snapshots": {
    "revision-id": {
      "plugins": []
    }
  }
}

The control plane must retain a revision in this catalog while any published UserConfig.plugin_bindings can reference it. Targets with no plugins are omitted; a revision is required whenever a target binding exists. There is no version 2 fallback.

The Control Plane publisher in IBM/mcp-context-forge#6252 must publish this catalog and matching effective-binding revisions before runtime plugins are enabled.

CPEX alignment

• CPEX documents find_plugin_entries plus invoke_entries as the external-orchestrator path and recommends caching the resolved entries: https://docs.rs/cpex/latest/cpex/struct.PluginManager.html#method.find_plugin_entries
• CPEX documents that the first lifecycle hook receives None and later hooks receive the returned context table: https://docs.rs/cpex-core/0.2.2/cpex_core/context/struct.PluginContextTable.html

Verification

• cargo nextest run --workspace --all-features (238 passed, 13 skipped)
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo deny check
• cargo shear
• cargo fmt --all -- --check
• signed commit hooks, including IBM Detect Secrets

Closes IBM/mcp-context-forge#6253