test(mouse-selection-keys): cover drag, copy, and non-terminal-yield branches

[dormouse-bot]

The nightly survey flagged handle-mouse-selection-keys.test.ts as covering only two of the handler's paths — the editable-target paste guard and the xterm-helper-textarea paste — leaving the module's core selection logic untested. This adds cases for the untested branches in handle-mouse-selection-keys.ts, so a regression in any of them fails loudly instead of shipping green.

New cases:

• Non-terminal yield — a surface whose nav.paneParams reports surfaceType: 'agent-browser' returns false and does not paste, matching the documented invariant that browser/iframe surfaces own their clipboard keys.
• Drag: token extend — e with a hintToken active calls extendSelectionToToken(sid, hintToken).
• Drag: Escape — clears the selection via setSelection(sid, null).
• Drag: key swallowing — a non-Alt key during a drag is swallowed (preventDefault) and handled, while Alt is handled but left to reach the OS (the block-selection modifier).
• Copy — Cmd+C calls copyRaw and Cmd+Shift+C calls copyRewrapped, each followed by flashCopy with the matching kind.

To drive the drag/selection shape the handler reads, the test now mocks lib/mouse-selection (the per-id module-state store) alongside the existing clipboard and platform mocks. The two original tests are unchanged.

Test-only change. vitest run (7 passed) and tsc -b both pass locally.

Surfaced by the nightly code-quality survey.

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