feat: add LRU eviction with entry limit to SchemaStore to prevent unbounded memory growth (fixes #604)

[hanhan761]

Summary

SchemaStore._analyzers Map grew unbounded — no eviction strategy existed. In long-running sessions with many collections, memory usage increased without limit.

Changes

• LRU eviction: Added _accessOrder tracking (last-access timestamps) and _evictIfNeeded() that removes the least-recently-used entry when the cache exceeds the limit
• Default limit: 100 entries (DEFAULT_MAX_ENTRIES)
• Access tracking: All read methods (hasSchema, getKnownFields, getSchema, getDocumentCount) and writes (addDocuments) update access timestamps
• Eviction telemetry: evictionCount and maxEntries measurements in schemaStore.stats event for monitoring
• Cleanup: _accessOrder cleared in clearSchema, clearCluster, clearDatabase, reset, and dispose

Design

• LRU policy (access-order-based) is simpler and more predictable than time-based eviction for a cache that receives bursty access patterns
• Single _evictIfNeeded() call path in addDocuments() (only when a new entry is created) — no periodic timer needed
• Zero behavior change for sessions with fewer than 100 collections

Issue

Fixes #604

Verification

• Single-file change (src/documentdb/SchemaStore.ts, +81/-7)
• Existing read/write APIs unchanged (same signatures, same return values)
• _accessOrder cleaned up in all existing lifecycle methods

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds an LRU-based soft cache limit to SchemaStore to prevent unbounded schema analyzer growth during long-running VS Code sessions, and reports eviction metrics via telemetry.

Changes:

• Introduces LRU tracking (_accessOrder) with a default max entry limit and eviction counter.
• Touches cache entries on reads/writes and evicts least-recently-used entries when above the limit.
• Publishes new telemetry measurements (evictionCount, maxEntries).

[tnaum-ms]

Maintainer update

I pushed a few changes to this branch to get it merge-ready (commit on top of your work):

Changes

• Lowered the cache cap from 100 to 50 collections (DEFAULT_MAX_ENTRIES = 50).
• Made eviction telemetry reliable: an eviction now flags a stats change so the cumulative evictionCount is flushed promptly in the schemaStore.stats event. Cache size is reported via currentCollectionCount / maxCollectionCount, alongside evictionCount and maxEntries, so we can monitor both the live map size and how often eviction is triggered.
• Added unit tests covering: the 50-entry cap, least-recently-used eviction order, read-as-recent-use (reads refresh recency), re-adding an evicted collection, and telemetry reporting.

Validation: prettier, lint, build, and the full Jest suite (2000 tests) all pass locally.

Thanks again for this, @hanhan761. The approach (insertion-order LRU with a single eviction path in addDocuments) is clean and well-scoped.