Test-only proof: wide-title tab drag regression

[austinywang]

Expected-red CI proof for the first commit of #215.

This draft intentionally contains only the behavioral regression tests from 0dc3b6d; the strip-wide destination and resolver are absent, so CI must fail. The production fix remains in #215.

Related: manaflow-ai/cmux#10033

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Summary by cubic

Adds failing regression tests that prove the wide-title tab drag bug and the need for a strip-wide tab-bar drop destination that resolves insertion slots over tabs and trailing chrome. Intentionally test-only; CI will fail until the implementation lands. Related to issue #10033.

• Refactors
  • Added TabBarDropHandlerTests, TabDropIndexResolverTests, and TabWideTitleDragTests to cover insertion index resolution (including wide tabs), same/cross-pane moves, live-drag vs pasteboard behavior, and destination hit-testing.
  • Removed TabDropDelegateSamePaneFallbackTests and updated BonsplitTests to assert a strip-wide drop destination spans rendered tabs and trailing chrome.

^{Written for commit 0dc3b6d. Summary will update on new commits.}

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