Preserve MCP server session isolation

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Summary

• propagate the server-level --session value to MCP-spawned CLI subprocesses
• preserve typed per-tool session overrides
• add regression coverage and document the server default behavior

Impact

Previously, agent-browser --session project-a mcp silently ran tool calls without a typed session in the global default session. Multiple MCP servers could therefore share cookies, storage, and restore state despite being launched with distinct session names.

Validation

• targeted MCP session regression test
• full Rust test suite: 897 unit tests and 2 integration tests passed
• cargo fmt -- --check
• cargo clippy (passes with three existing warnings in untouched files)
• end-to-end MCP stdio smoke checks for inherited defaults and per-tool overrides