chat: simplify symbol reference cache to an array (#300489)

* chat: simplify symbol reference cache to an array

Replaces the Map-based symbol reference cache with a simple array that
keeps only the last 3 copied symbols. This removes the 15-second TTL
expiration logic and relies on insertion order instead.

- Changed symbolReferenceCache from Map to array
- Removed symbolCacheTtlMs constant and pruning logic
- Added symbolCacheMaxSize constant for max entries
- Simplified cache lookups with array.find()
- Automatically evicts oldest entry when exceeding capacity

(Commit message generated by Copilot)

* comment

* skip flakey test, suggested by Megan