fix(extensions): reject non-callable VCS operation watch hooks

[edenbuilds]

Summary

registerVcsAdapter validates that an operation's load field is callable, but not the optional watchSignature / watchPlan hooks. An untyped extension can register:

{
  load: async () => result,
  watchSignature: "not a function",
}

Registration currently accepts this. The watchSignature && {...} spread guard in toInternalVcsOperation treats any truthy value as present, wraps it, and the resulting TypeError only surfaces once watch planning invokes the hook — well after registration.

Fix

toInternalVcsAdapter now requires watchSignature and watchPlan to be absent or callable before accepting an operation, matching the existing behavior for a non-callable load (the whole operation entry is dropped, so lookups report "not supported" instead of failing mid-review).

Test plan

• Added an operation with a non-callable watch hook is dropped, not wrapped to publicApiRobustness.test.ts, mirroring the existing load coverage.
• Confirmed the new test fails on current main (TypeError path) and passes with the fix.
• bun test packages/hunk/src/extensions/runExtension.test.ts — 51 pass, 0 fail.
• bun run typecheck, oxfmt --check, oxlint --deny-warnings all clean on the changed files.
• Added a patch changeset.

Fixes #763

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