Pin MCP runtimes to model steps

[jif-oai]

Why

An MCP refresh can replace the session's current manager while a model step is still running. The step must execute calls through the same manager whose tools it advertised.

Boundary

current session MCP runtime
          |
          | capture once for this model step
          v
StepContext.mcp
  - exact MCP config
  - exact connection manager
  - exact runtime environment context

pub struct McpRuntimeSnapshot {
    config: Arc<McpConfig>,
    manager: Arc<McpConnectionManager>,
    runtime_context: McpRuntimeContext,
}

Example

step A captures runtime A and advertises A's tools
refresh publishes runtime B
step A tool call -> runtime A
next step        -> runtime B

Capturing the snapshot is only an Arc clone. It does not restart MCPs or make an RPC.

What changes

• Captures one MCP runtime in StepContext.
• Uses it for tool planning, tool calls, resources, approvals, connector attribution, and elicitation.
• Publishes replacement runtimes atomically.
• Lets an old runtime live only while an in-flight step or request still holds its Arc.

Most of this diff is mechanical routing from the session-global manager to step_context.mcp; it does not introduce selected-plugin discovery yet.

What does not change

• No plugin or extension migration.
• No new MCP cache policy.
• No environment file watching.
• No client sharing between separate managers.

Stack

1. Extension-owned World State sections.
2. Project executor skills through World State.
3. This PR: pin one MCP runtime to each model step.
4. Project selected MCP/app/connector metadata by environment availability.
5. One end-to-end integration scenario.

[chatgpt-codex-connector]

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