feat(runs): prune empty setting messages before storing

[davidlin20dev]

Tracking issue

Related to #7775. This closes task 2.1 (proto to protojson transformers), in a smaller form than the RFC specced. Related to #7932.

Why are the changes needed?

Task 2.1 asked for two transformers around stored protojson: Hydrate on read and PruneInherited on write. Most of that job turned out to be built into the serialization layer already:

• Hydrate is a no-op. INHERIT is the proto3 zero value, and the nil-safe getters return it for absent fields, so a field missing from stored JSON already reads back as INHERIT. So there is nothing to implement.
• Prune is mostly free. protojson.Marshal already omits zero scalars and nil messages. The one gap: a client can spell INHERIT as an empty message ("defaultQueue": {}), and that form is stored verbatim and echoed back by GetSettingsForEdit.

The empty-message form changes no behavior (both forms read back identically), but it makes stored bytes depend on client phrasing. The Phase 2 exit criteria pins exact response bytes against settings_customer_flow.md, which needs one canonical stored form. This PR adds the missing piece and drops the rest of 2.1.

What changes were proposed in this pull request?

pruneSettings in runs/service/settings_prune.go, called by CreateSettings and UpdateSettings right before marshaling:

• Walks the message generically with protoreflect and clears every message field with nothing populated inside it. New Settings fields are covered automatically.
• Children are pruned before their parent is judged, so a parent emptied by pruning is removed too.
• UNSET survives: its state field is populated, so only truly empty messages are removed.
• Side effect worth noting: Create and Update echo the request settings in their responses, so after pruning the echo matches the stored bytes exactly.

How was this patch tested?

• Table tests in settings_prune_test.go: removal at every nesting depth, an empty map wrapper, UNSET kept, a value kept next to an empty sibling, clean settings unchanged, and a nil guard.
• Assertions use proto.Equal, since protojson output bytes are deliberately unstable.
• Existing settings service tests pass unchanged, so no read path observes the difference.

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