refactor: decouple MCP policy construction from escalate server

[bolinfest]

Why

The current escalate path in codex-rs/exec-server still had policy creation coupled to MCP details, which makes it hard to reuse the shell execution flow outside the MCP server. This change is part of a broader goal to split MCP-specific behavior from shared escalation execution so other handlers (for example a future ShellCommandHandler) can reuse it without depending on MCP request context types.

What changed

• Added a new EscalationPolicyFactory abstraction in mcp.rs:
  • crate-relative path: codex-rs/exec-server/src/posix/mcp.rs
  • https://github.com/openai/codex/blob/main/codex-rs/exec-server/src/posix/mcp.rs#L87-L107
• Made run_escalate_server in mcp.rs accept a policy factory instead of constructing McpEscalationPolicy directly.
  • https://github.com/openai/codex/blob/main/codex-rs/exec-server/src/posix/mcp.rs#L178-L201
• Introduced McpEscalationPolicyFactory that stores MCP-only state (RequestContext, preserve_program_paths) and implements the new trait.
  • https://github.com/openai/codex/blob/main/codex-rs/exec-server/src/posix/mcp.rs#L100-L117
• Updated shell() to pass a McpEscalationPolicyFactory instance into run_escalate_server, so the server remains the MCP-specific wiring layer.
  • https://github.com/openai/codex/blob/main/codex-rs/exec-server/src/posix/mcp.rs#L163-L170

Verification

• Build and test execution was not re-run in this pass; changes are limited to mcp.rs and preserve the existing escalation flow semantics by only extracting policy construction behind a factory.

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• refactor: normalize unix module layout for exec-server and shell-escalation #12556
• -> refactor: decouple MCP policy construction from escalate server #12555