[CF-DATAPLANE] logLevel and request-scoped notification separation

[gandhipratik203]

Parent: #5681

Depends on: #5679
Related: #5953

Goal

Implement and verify MCP 2026-07-28 request-scoped notification behavior for notifications/message and io.modelcontextprotocol/logLevel in the Rust dataplane.

Request-scoped notifications must stay on the response stream for the originating request and must not be routed through subscriptions/listen.

Scope

• Verify what RMCP 3.0.1 already enforces for request-scoped notification routing.
• Plumb io.modelcontextprotocol/logLevel from request _meta through the gateway path.
• Suppress notifications/message when logLevel is absent.
• Preserve request-scoped notifications/progress behavior on the originating request stream.
• Add tests proving notifications/progress and notifications/message do not appear on subscriptions/listen streams.

Acceptance criteria

• A backend notifications/message emitted for a request with logLevel is delivered only on that request's response stream.
• A backend notifications/message emitted for a request without logLevel is suppressed.
• notifications/progress remains scoped to the originating request stream.
• subscriptions/listen streams never carry request-scoped notifications/progress or notifications/message.
• Tests identify which behavior is provided by RMCP and which behavior is implemented in the gateway.

Non-goals

• List-change notification relay; tracked in [CF-DATAPLANE] Relay MCP list-changed notifications through gateway #5346.
• Resource update notification relay through subscriptions/listen; tracked in [CF-DATAPLANE] Relay MCP 2026-07-28 resource update notifications through subscriptions/listen #5615.
• Upstream subscriptions/listen management; tracked in [CF-DATAPLANE] Upstream subscriptions/listen management for modern backends #6118.
• Control-plane, UI, or management API changes.