Centralize work-pool result storage eligibility

[zzstoatzz]

this PR centralizes the rule that decides when a work-pool result storage default is allowed to apply for ad-hoc submission.

This is a small phase-1 Storage Defaults cleanup. Work-pool result storage is the only partial default layer we have today, and the project needs its precedence rule to be predictable before we add workspace or server defaults.

details

• adds a private helper in prefect.results for one shared question: does this flow already have result storage configured in a way that works for remote result retrieval?
• uses that helper from both ad-hoc submission paths: InfrastructureBoundFlow.submit_to_work_pool() and BaseWorker._submit_adhoc_run()
• keeps the actual work-pool override behavior explicit in those callsites instead of introducing a generic source-selection abstraction
• adds focused tests for the helper and preserves the existing behavior tests for both submission entrypoints

Why this matters for the project:

• explicit flow-level storage still wins
• the work-pool default remains the next lower-precedence option in these ad-hoc submission paths
• the rule for when work-pool storage is allowed to step in now lives in one place instead of two copies

Tests:

• uv run pytest tests/results/test_result_store.py -k "RemoteResultStorageConfiguration or update_for_flow"
• uv run pytest tests/test_infrastructure_bound_flow.py -k "test_submit_uses_work_pool_result_storage_block or test_submit_to_work_pool_does_not_override_explicit_flow_result_storage"
• uv run pytest tests/workers/test_base_worker.py -k "test_submit_uses_work_pool_result_storage_block or test_submit_does_not_override_explicit_flow_result_storage"
• uv run ruff check src/prefect/results.py src/prefect/flows.py src/prefect/workers/base.py tests/results/test_result_store.py

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing codex/oss-7740-unified-result-storage-resolution (c98f8fe) with main (d1d0f4d)}