Fix cancel teardown shutdown crash

[BrainSlugs83]

Summary

Fixes a shutdown-time 0xC0000005 access violation seen after model unload when tearing down Foundry Local from an embedded host.

The crash was reproducible during process shutdown after cancellation/unload. The root issues were:

• the OGA log callback could remain registered until after ORT/EP teardown;
• SessionManager::CancelAll() was effectively a no-op for active chat work;
• shutdown could unload models before in-flight web/session work had fully drained;
• ORT GenAI/EP global teardown is currently unsafe in this embedded disposal path.

Fix

• Clear the OGA log callback before model unload / ORT-related teardown.
• Wire cancellation through SessionManager, Session, and ChatSession so active work can be signaled.
• Reorder shutdown to reject new loads, cancel, stop/reap web-service work, drain sessions, then unload models.
• Keep registered EP libraries and the ORT environment alive for process lifetime to avoid the late teardown AV after sessions/models are already disposed.
• Add teardown trace points around model unload and manager disposal to make future shutdown failures diagnosable.

Validation

Validated by the downstream repro harness and unit suite:

• Reproduced the original shutdown crash (0xC0000005) before the fix.
• Re-ran the same harness after the fix: HARNESS_EXIT=0.
• Ran C++ unit tests: 111 tests passed.

Scope

This PR intentionally contains only the upstreamable cancel/teardown crash fix.

A downstream-only telemetry preference (TelemetryLogger replaced by NullTelemetry) exists in the local shippable branch, but is intentionally excluded from this PR because it is a product/build preference rather than a general Foundry Local bug fix.

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