fix(framework): Preserve output before runtime cleanup

[tanertopal]

What changed

• allow an exit callback to run after callbacks that are already registered
• register runtime-environment deletion as low-priority cleanup
• preserve normal LIFO behavior for all other callbacks

Why

Runtime dependency installation registered filesystem cleanup after the executor output handler. LIFO execution could therefore delete the environment before final task status and context were reported.

Stack

Umbrella/reference: #7895 — Prevent task executor shutdown deadlock

• Previous: #7920 — Stop logging executors before completion
• This PR: fix(framework): Preserve output before runtime cleanup #7921 — Preserve output before runtime cleanup
• Next: #7922 — Interrupt Fleet retries during teardown

Merge the stack in order. After the previous PR merges, retarget this PR to main.

Validation

• pytest py/flwr/supercore/exit/exit_handler_test.py py/flwr/supercore/superexec/dependency_installer_test.py
• Ruff and Black on the changed modules
• git diff --check