[CF-DATAPLANE][CPEX] Run resource hooks for resource reads

[lucarlig]

What this accomplishes

This PR lets the external dataplane apply CPEX policy to the complete resources/read lifecycle.

Before a backend resource is fetched, plugins receive its canonical backend-local URI and can allow or deny the request. After the backend responds, plugins can inspect, redact, or transform both text and binary resource content before it reaches the MCP client. Context created by the pre-fetch hook is preserved for the matching post-fetch hook, even if the configured runtime is reloaded while the backend call is in flight.

The result is that resource reads receive the same policy-enforcement boundary already available to tool and prompt calls. The bundled secrets-detection plugin demonstrates this by redacting a password resource for both direct and namespaced resource URIs.

Safety behavior

• Plugins cannot change the canonical resource route, URI, MIME type, item identity, CMF schema version, or channel.
• MCP base64 blobs are decoded for plugin inspection and safely encoded after modification.
• Content left unchanged by plugins is returned exactly as supplied by the backend, including metadata and the original blob representation.
• Unsupported transformations, invalid backend blobs, and missing or corrupted post-hook lifecycle state fail closed instead of returning uninspected content.
• Calls allocate no resource correlation state when neither resource hook is configured.

Scope

This keeps the existing global CPEX configuration version and request-local PluginContextTable behavior. Scoped plugin bindings, trusted typed extensions, catalogs, caches, and runtime revisions are intentionally excluded pending the producer contract in IBM/mcp-context-forge#6252.

Part of IBM/mcp-context-forge#6253.

Protocol-specific coverage for MCP 2026-07-28 and 2025-11-25 is isolated in stacked PR #129.

Verification

• cargo nextest run --locked --workspace --all-features
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo deny check advisories bans licenses
• cargo shear --check-test-targets --deny-warnings --locked
• repository pre-commit suite