feat(studio): add studio_inspect, so an agent reads before it writes

[miguel-heygen]

What

studio_inspect returns everything about one element in a single call: resolved styles, text fields, box, data attributes, GSAP animations, and what the element will and will not accept.

Also extracts the shared WebMCP test fixtures into webmcpTestUtils.ts, which removes the duplication that was accumulating across the tool test files and the as HTMLElement casts that came with it.

Stacked on #3516.

Why

The point is to prevent a failed write, not to satisfy curiosity. can.reasonIfDisabled comes through verbatim from Studio's own capabilities, so an agent that reads first should never attempt an edit the element would refuse. That matters more here than in a normal API, because a refused write in Studio is often silent.

How

Three things it refuses to get wrong:

Animations are reported only for the current selection, because that is the only element Studio parses them for. Attributing them to any other element would be reporting the wrong element's motion, which is worse than reporting none. When a handle names something else the list is empty and animationEditingBlocked says why.

animationEditingBlocked also carries the two states where animation editing is off entirely, multiple timelines and an unsupported timeline pattern. Both live on the selection context. Learning them from a read costs one call; learning them from a failed write costs a retry loop.

Inspecting does not change the selection. It is a read, and stealing the human's selection would be a side effect they did not ask for. There is a test asserting applySelection is never called.

Nothing selected and no handle given is a failure, not an empty result. An empty result would assert "this element has nothing", which is a different and false claim.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)

8 tests in inspectTools.test.ts, and the whole WebMCP suite re-run after the fixture extraction:

• Resolved styles come back, not authored ones.
• Capabilities and reasonIfDisabled pass through verbatim.
• Animations list for the current selection, with ease and id.
• Both animationEditingBlocked reasons are asserted separately.
• A handle naming a different element gets an empty animation list and a reason, rather than the selection's animations.
• Inspecting a handle does not call applySelection.
• No selection and no handle is a failure with a hint, not an empty result.
• Unknown handle and unmounted preview produce distinct reasons.

WebMCP suite 78 passing. Full package suite 4563 passing across 411 files. bunx tsc --noEmit clean, bunx oxlint clean, bunx fallow audit --fail-on-issues clean.

Still no browser verification anywhere in this stack; that is the final unit.