## Why
`ToolExecutor::exposure()` describes a tool's preferred exposure, while the host
may apply step-specific policy when building a tool plan.
## What changed
- Store each runtime together with its effective exposure in `ToolRegistry`.
- Apply MCP, namespace, and collaboration-tool exposure overrides directly to
registry entries instead of wrapping runtimes.
- Use the registry-owned exposure when building model-visible specs, deferred
tool search, code-mode executors, and parallel-call support.
## Testing
- Cover that a hidden MCP tool remains routable but is not eligible for parallel
tool calls.
GitOrigin-RevId: 1bc2760add8d993f477954269fa13681bfcc2444