perf: Optimize workflow name dropdowns with /names API

[akhilpathivada]

Resolves #1014

Summary

• Add GET /metadata/workflow/names endpoint that returns distinct workflow names via SELECT DISTINCT name — avoids loading json_data column and JSON deserialization entirely
• Update UI hooks (useWorkflowNames, useWorkflowNamesAndVersions) to call lightweight endpoints directly instead of deriving from useWorkflowDefs() which loads all full definitions
• On pages with both dropdowns (e.g., Workbench), /names loads the name dropdown and /names-and-versions is fetched in parallel for the version dropdown. Both return only lightweight metadata — no full definition bodies are loaded until the user picks a specific version via GET /metadata/workflow/{name}?version=N
• On pages with only a name dropdown (e.g., Execution Search, Task Search), only /names is
  called — the version summaries are never fetched at all

Changes

Backend

• MetadataDAO: Added default getWorkflowNames() method — backward-compatible fallback for persistence modules that don't override it
• PostgresMetadataDAO: Optimized override using SELECT DISTINCT name FROM meta_workflow_def ORDER BY name — index-only scan, zero deserialization
• MetadataService / MetadataServiceImpl: Added getWorkflowNames() delegation
• MetadataResource: New GET /metadata/workflow/names REST endpoint

UI

• useWorkflowNames(): Now calls /metadata/workflow/names directly
• useWorkflowNamesAndVersions(): Now calls /metadata/workflow/names-and-versions directly
• Both hooks previously derived their data from useWorkflowDefs() → GET /metadata/workflow
  which loaded all full WorkflowDef objects

Tests

• MetadataServiceTest: Unit test for getWorkflowNames()
• PostgresMetadataDAOTest: Integration test for the optimized DAO method
• MetadataResourceTest: REST endpoint test
• Cypress: Added fixture and intercept for /metadata/workflow/names

Backward Compatibility

• No database schema changes or migrations
• No existing endpoints modified or removed
• MetadataDAO.getWorkflowNames() is a default method — existing DAO implementations (Redis, MySQL, etc.) continue to work without changes
• Deploy backend before (or together with) UI

Future Scope

• Introduce a per-workflow versions endpoint (e.g., GET /metadata/workflow/{name}/versions) to fetch version details lazily only when a user selects a specific workflow, instead of loading all workflows' version summaries upfront via /names-and-versions

Test Plan

• ./gradlew spotlessApply passes
• ./gradlew test passes (core, rest, postgres-persistence)
• Cypress UI tests pass
• Verify workflow name dropdown loads on Execution Search page
• Verify workflow name dropdown loads on Workbench page
• Verify version dropdown populates after selecting a workflow name on Workbench
• Verify workflow name dropdown loads on Start Workflow page
• Verify no regression on Workflow Definitions listing page (/definitions/workflow)