ENT-3868: Improve persistent class logging

[nickanderson]

Summary

• Fix pre-existing memory leak of existing_info in EvalContextHeapPersistentSave (missing free() on the fall-through and early-return paths)
• Replace the generic "Updating persistent class '%s'" log message with context-aware messages that distinguish between:
  • Creating a new persistent class (no prior record in the DB)
  • Resetting a timer (existing record with RESET policy)
  • Updating an existing record (expired PRESERVE or changed tags)

This helps diagnose timer_policy behavior from verbose logs, which was the remaining gap identified in ENT-3868.

Before

Updating persistent class 'my_class'

After

Creating persistent class 'my_class' (10 minutes, policy reset)
Resetting persistent class 'my_class' timer to 10 minutes (was 7 minutes remaining)
Persistent class 'my_class' is already in a preserved state -- 7 minutes to go
Updating persistent class 'my_class' (10 minutes, policy preserve)

Changelog

Commit 1: Fix memory leak

existing_info was allocated via xcalloc but never freed on the fall-through path (after the PRESERVE early-return check) or on the error early-return path. Added free(existing_info) in all code paths.

Commit 2: Improve logging

Moved logging into the branches where the existing/new state is already known, replacing the single generic message with descriptive alternatives.

Test plan

• Verify build succeeds
• Write a policy with persistence => "10" on a classes promise, run cf-agent -Kv twice — first run should log "Creating", second should log "Resetting"
• Write a policy using timer_policy => "absolute" in a classes body on a files promise, run twice — should log "Creating" then "already in a preserved state"

Ticket

ENT-3868