Add server default result storage config surface

[zzstoatzz]

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

What changed

• adds a ServerDefaultResultStorage schema for representing the configured block document id
• adds /admin/storage read, update, and clear endpoints
• adds client methods for reading, updating, and clearing the server default result storage setting
• validates that the configured block exists and supports write-path
• keeps storage-default-specific validation and persistence in a dedicated server.models.storage_defaults module instead of leaving that logic inline in the route
• syncs the generated ui-v2 OpenAPI client for the new admin surface

Why this is a separate slice

This PR only adds the configuration surface for storing a server default result storage block. It does not change runtime result-storage resolution yet.

That separation keeps the persisted setting and API contract reviewable on their own before the follow-up runtime behavior work lands.

This still matches the Cloud backend split: Cloud has a separate storage_defaults model module too, but its propagation and materialization logic lives in later slices. OSS is intentionally stopping at the persisted setting and API contract in this first PR.

Generated client note

ui-v2/src/api/prefect.ts includes the expected /admin/storage additions for this feature.

It also still includes the unrelated IntervalSchedule.interval change from number to number | string. I reproduced that by running npm run service-sync on a clean origin/main worktree, so it appears to be existing generator drift rather than a storage-defaults-specific change.

Verification

• uv run pyright --ignoreexternal --verifytypes prefect
• uv run pytest tests/server/api/test_admin.py tests/server/models/test_storage_defaults.py tests/client/test_prefect_client.py -k 'server_default_result_storage'
• uv run pytest tests/server/models/test_configuration.py tests/server/models/test_storage_defaults.py
• uv run ruff check src/prefect/server/api/admin.py src/prefect/server/models/__init__.py src/prefect/server/models/configuration.py src/prefect/server/models/storage_defaults.py tests/server/api/test_admin.py tests/server/models/test_configuration.py tests/server/models/test_storage_defaults.py tests/client/test_prefect_client.py
• repo hooks passed during commit and push, including mypy and Sync UI v2 OpenAPI

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

---

_{Comparing codex/storage-default-config-surface (e7f39ad) with main (1c970c3)}

[desertaxle]

Why is this optional? Should it be required since you're also including an endpoint for deletion?

[desertaxle]

I was looking into this configuration cache, and it looks like it's in memory. We might want to make that configurable because changes to the default storage might not propagate in multi-server environments. That feels like a follow-up PR, though.