Two related tidy-ups to query-cache dependent-table handling.

[rob-bygrave]

This is a replacement for #3150

1. Read dependentTables from the query plan (single source of truth)

OrmQueryRequest accumulated query-cache dependentTables into a field via
addDependentTables(...), fed from per-CQuery dependentTables() helpers. The
CQueryPlan already owns this set, so the copying is redundant.

• putToQueryCache(...) now reads dependentTables directly from the request's CQueryPlan at cache-put time.
• Removed the OrmQueryRequest.dependentTables field and addDependentTables(Set).
• Removed the now-unused dependentTables() helpers from CQuery, CQueryRowCount and CQueryFetchSingleAttribute (and their Set imports).

This is behaviour-preserving: each CQuery is built with exactly the plan stored
under request.queryPlanKey, so request.queryPlan().dependentTables() is the
same set the per-CQuery helpers returned.

2. Fix a query-plan trim race that could null the plan at put time

A freshly built CQueryPlan was stored in the plan cache with
lastQueryTime == 0, making it immediately eligible for trimQueryPlans
(runs every 60s; TTL default 300s) during its first execution. If the trim
fired mid-execution, request.queryPlan() could return null at put time — and
a cache entry with null dependentTables is never invalidated by table
modifications (TableModState.isValid), i.e. latent stale data.

• CQueryPlanStats now initialises lastQueryTime to construction time, so a new plan is only trim-eligible after a genuine TTL idle. trimQueryPlans is the sole consumer of lastQueryTime(), so this is safe.
• putToQueryCache(...) skips the put when the plan is null (fail safe rather than caching an un-invalidatable entry) for the residual pathological case (a single query running past the TTL on first execution).

Tests

• Added testFindSingleAttributeOnDependent and testFindListOnDependent to TestQueryCacheTableDependency, asserting cache-hit then dependent-table invalidation for the single-attribute and findMany paths (the count path was already covered).
• All cache/query tests pass: 69 in ebean-test org.tests.cache.**, 929 in ebean-core cache + query packages.