feat(pj_datastore): add ObjectStore::findTopic(dataset_id, name)

[pabloinigoblasco]

Summary

Add a pure-lookup API to ObjectStore:

std::optional<ObjectTopicId>
ObjectStore::findTopic(DatasetId, std::string_view) const noexcept;

Returns the existing handle if a topic with the given (dataset_id, topic_name) is registered, nullopt otherwise. Does not mutate the store.

registerTopic keeps its strict semantics: it still returns unexpected("topic already registered: ...") when a duplicate is attempted. Callers that need find-or-register semantics compose the two explicitly.

Why

pj_datastore/docs/OBJECT_STORE_DESIGN.md documents the responsibility split:

• DataSource plugins register object topics on the source-side path (pj.source_object_write.v1).
• Host runtime resolves an existing id when binding parser-side write surfaces (pj.parser_object_write.v1).

In other words: the source owns registration; the host looks up.

Until now the host had no lookup-only API, so it called registerTopic from its parser-binding flow. When a source had already registered the same topic via its own service, the second caller failed with "topic already registered: ...", the source-side pushLazy path went silently unwired, and blob channels lost retention.

findTopic lets the host honour the documented contract: bind to an existing id without creating one.

Diff

File	Change
pj_datastore/include/pj_datastore/object_store.hpp	declare findTopic
pj_datastore/src/object_store.cpp	implement (std::shared_lock on the store mutex; linear scan over topics_ keyed by (dataset_id, name), same shape as registerTopic's duplicate check)
pj_datastore/tests/object_store_test.cpp	two new tests (see below)

Total: +32 / −0 across 3 files.

Tests

• FindTopicReturnsRegisteredId — happy path lookup after register.
• FindTopicMissingReturnsNullopt — explicit miss returns nullopt.
• DuplicateRegistrationFails (existing) is left untouched — registerTopic's strict contract is preserved.

Backward compatibility

Pure addition. Existing callers of registerTopic are unchanged. No semantic changes to existing APIs.