Delete leaf device identities in one-time test teardown

[damonbarry]

Summary

This change updates end-to-end test cleanup so leaf device identities are deleted during suite teardown instead of inside each individual test cleanup path.

Problem

In nested end-to-end scenarios, leaf identity deletion can be slow enough to consume the per-test timeout budget.
For RouteMessageL3LeafToL4Module specifically, test execution could succeed functionally but still time out while deleting leaf identity in the test-finally block.

What changed

• Added a dedicated leaf cleanup collection on test context.
• Registered created leaf devices in that collection at creation call sites across the E2E test suite.
• Removed per-test leaf identity deletion from test-finally blocks.
• Extended one-time teardown to delete all queued leaf identities using the teardown timeout token.

Why this approach

• Aligns leaf cleanup behavior with existing edge device cleanup strategy.
• Keeps test timeout focused on test behavior, not cloud identity teardown latency.
• Reduces flakiness in nested topologies where cleanup can be slower.

Scope

Applied to all current E2E test call sites that create leaf devices, including:

• Device tests
• Device with custom certificates tests
• Plug and Play tests
• X509 device tests

Validation

• Verified that the nested end-to-end tests pass

Expected impact

• Fewer false-negative timeouts caused by cleanup latency.
• No functional change to test assertions or message validation logic.
• Leaf identity deletion still occurs, but now under teardown timeout budget.

Risk and mitigation

• Risk: if teardown is interrupted, leaf identities may remain temporarily.
• Mitigation: cleanup remains centralized and bounded by teardown timeout; behavior matches existing edge identity cleanup pattern.