WebMCP Gatherers Incompatible with Snapshot Mode

WebMCP audits rely on CDP WebMCP domain. Specifically [WebMCP.enable](https://chromedevtools.github.io/devtools-protocol/tot/WebMCP/#method-enable) allows for registered tools to be returned.

`WebMCP.enable` is currently called in [`startInstrumentation`](https://github.com/GoogleChrome/lighthouse/blob/3608d82ba7ee4912e5ba9af9eeb178168ec08b62/core/gather/gatherers/webmcp.js#L75) which isn't supported in snapshot mode.

We should probably remove snapshot mode form WebMCP audits and then either
* modify CDP to add a call to return all registered tools or
* run `WebMCP.enable` outside of `startInstrumentation`

Start in core/gather/gatherers/webmcp.js at startInstrumentation and compare the WebMCP.enable call with snapshot-mode handling. Read the linked CDP WebMCP.enable documentation and the WebMCP audit behavior. Done means snapshot-mode compatibility and registered-tool collection have an explicit resolution.

Written by the indexing model from the issue text.