The ti:self scope check at security.py rejects requests where the JWT subject does not match the task_instance_id in the path. This is the cross-task isolation guarantee for any router that opts into ti:self (e.g. /task-instances, /hitl, and the upcoming task state endpoints from AIP-103). Until now the check was uncovered.

Adds three tests mirroring the existing TestTokenTypeScopeEnforcement pattern: matching subject is accepted, mismatched subject returns 403, and routes without ti:self ignore subject mismatches.

Why

execution API uses the ti:self scope to enforce that a task can only access its own resources — JWT subject must equal the task_instance_id in the request path. The check lives at https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/api_fastapi/execution_api/security.py#L185-L193 and is what stops a task from reading or writing another task's state, HITL responses, etc.

I noticed thattest_security.py only tested token-type enforcement (workload vs execution); the cross-TI check was relying on no one breaking it.

While working on AIP-103 (task / asset state endpoints), I needed this guarantee to be defended before claiming that "a task can only access its own state" is true. Filing this separately so the coverage benefits every router using ti:self, not just the state endpoints.

Summary

Adding tests for three cases:

• Matching JWT subject is accepted (200)
• Mismatched JWT subject is rejected (403, "does not match")

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