Add server default result storage config surface

[zzstoatzz]

this PR adds the OSS config surface for a server-level default result storage setting.

It includes:

• server schema, model, and admin API for reading/updating/clearing the configured block
• client schema/routes/methods for the same surface
• validation that the configured block exists and supports result-storage writes
• focused API/model/client test coverage

This is the first foundational slice of the Storage Defaults work. It intentionally does not change runtime resolution yet, so the new setting is not consumed until the follow-up runtime PR lands.

A note on ui-v2/src/api/prefect.ts:

• the new admin route/schema additions are expected from the generated client sync
• the heartbeat_frequency and interval diffs are not from this feature specifically; they were reproduced by regenerating the client on a clean origin/main, so they appear to be existing generator drift rather than storage-default logic

Verification:

• uv run pytest tests/server/api/test_admin.py tests/server/models/test_configuration.py tests/client/test_prefect_client.py -k "server_default_result_storage"
• uv run ruff check src/prefect/server/schemas/core.py src/prefect/server/models/configuration.py src/prefect/server/api/admin.py src/prefect/client/schemas/objects.py src/prefect/client/orchestration/routes.py src/prefect/client/orchestration/__init__.py tests/server/api/test_admin.py tests/server/models/test_configuration.py tests/client/test_prefect_client.py

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

---

_{Comparing codex/storage-default-config-surface (78a9f1a) with main (b89b43d)}