test(framework): Add k8s capacity cleanup proof

[msheller]

Summary

Adds a local k3d capacity/cleanup proof for the KubernetesExecutor launch path:

• adds capacity-cleanup-proof harness mode with active Pod budget 1
• seeds two deterministic ServerApp runs and records SuperExec capacity-wait evidence
• captures second TaskExecutor launch after capacity opens
• records completed TaskExecutor Pod and credential Secret cleanup before namespace deletion
• tightens verifier checks for two observed task records and remaining post-cleanup Pod evidence

Stack

Stacked on #7405 (codex/f7-k8s-evidence-hardening).

Evidence

Passing real k3d proof bundle:

/private/tmp/f7d-v2-capacity-cleanup-proof-lead-fixes-20260617-003

Saved verifier output:

/private/tmp/f7d-v2-capacity-cleanup-proof-lead-fixes-20260617-003/diagnostics/verifier-output.txt

Verifier result: Verification: PASSED.

Validation

$dev-format-and-test auto profile was run from the clean branch. It selected the fast gate because there were no uncommitted changed files.

Result: failed in test-fast on isort formatting checks:

• framework/py/flwr/supercore/superexec/executor/factory.py
• framework/py/flwr/supercore/superexec/executor/kubernetes_executor_harness_test.py

No source edits were made after the requested commit.

Additional implementation-thread validation before commit included targeted py_compile, bash -n, git diff --check, targeted Ruff, targeted pytest, and real k3d verifier passing.