test(context): add comprehensive frame manager test suite (STA-320)

Covers 37 tests across 13 previously untested public methods:
getFrame, getStack, getStatistics, getRecentFrames, deleteFrame,
getRecoveryReport, generateDigest, addContext, getActiveArtifacts,
extractConstraints, setQueryMode, runRecovery, validateProjectIntegrity.

Author: StackMemory Bot (CLI)