🤖 tests: pin the reserved-launch paths the startReservedAgentTaskLeased mutation pass left open #7761

A mutation pass over the new steps of the startReservedAgentTaskLeased split (stack 10: #7751, #7755, #7752, #7756-#7760) found behaviour that no test pins. These gaps existed before the split, which moved the code unchanged.

Can delete data (now covered)

• cancelMaterializedLaunch on a shared checkout. Fixed in 🤖 refactor: split startReservedAgentTaskLeased, part 5: stop latch, attempt stamp and desktop admission #7760: a test launches a shared, pre-identity row through the lease wrapper with a signal that aborts after the materialize step, and checks that the cleanup preserves the parent's checkout. The first version of this issue said only a worktree runtime could reach this path; the stack's advisor showed the local harness can.

Other gaps

• The drain after running, and the drain while the stop latch is still held: visible only as drain counts.
• The metadata emit after the stop-latch deferral and after the parent's runtime update: visible through workspaceService.emit, which no test checks.
• The adopt step moved after the desktop step: shows only for a pre-identity row that the desktop gate refuses.
• deferLaunchForStopLatch with allowMissing: false: differs only if the row is removed between the first read and the revert.

Trigger: after stack 10 merges, before more changes land in the reserved launch.

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