Make webhook lock tests causal

[cbusillo]

Summary

• replace private webhook test polling loops with bounded continuation-backed gates
• split the timing-dependent concurrent writer test into deterministic held-lock refusal and shared-state contracts
• retain and document the one snapshot-read sensitivity delay that lacks a production observer seam
• guarantee held-lock and spawned-task cleanup on throwing paths

Refs #578

Validation

• swift test --filter limitWarningWebhook (21 passed)
• 20 repeated focused stability runs passed
• lock-bypass mutation: removing sendTest locking failed the held-lock test on delivery, POST, and state assertions
• python3 scripts/context-panel-test-lanes.py time-command --lane routine-ci-swift -- swift test (891 passed)
• scripts/commit-gate.sh
• JetBrains changed-files inspection: GREEN, 0 findings
• Opus and Gemini exact-SHA final reviews approved 3ca57eb

Scope

• test-only change in Tests/ContextPanelCoreTests/LimitWarningTests.swift
• no production lock or webhook behavior changed