feat(agent-service): persist agent state to disk and rehydrate on startup

[bobbai00]

What changes were proposed in this PR?

Each agent was a stateful object in a process-local Map, never persisted, so a restart / crash / redeploy lost every agent and its conversation. This adds opt-in disk persistence for agents:

• TexeraAgent.toSnapshot() / restoreFromSnapshot() — capture and restore the durable state: the ReAct step tree, HEAD, settings, delegate metadata, and the workflow being edited. The short-lived user token and the recomputable execution-result cache are intentionally excluded. createdAt is preserved across restarts.
• AgentSnapshotStore (src/persistence/) — one JSON file per agent, atomic writes (temp file + rename), debounced coalescing of rapid updates, and loadAll() that skips unreadable/unsupported files.
• Server wiring — persists on create / clear / checkout / settings change and after each WebSocket turn; removes the file on delete; and rehydrates all agents on startup via rehydrateAgents. Enabled by AGENT_STATE_DIR; leaving it empty keeps the previous in-memory-only behavior.

Before:  restart -> agentStore empty            -> all agents + history lost
After:   restart -> loadAll() + rehydrate       -> agents restored from disk

Any related issues, documentation, discussions?

Closes #5267

How was this PR tested?

cd agent-service
bun test            # 110 pass, 0 fail
bun run typecheck   # clean
bun run format:check

New tests:

• agent/texera-agent-snapshot.test.ts — toSnapshot of a fresh agent, createdAt round-trip, restoreFromSnapshot restoring conversation/settings/workflow/delegate, a full JSON round-trip deep-equal, and version rejection.
• persistence/agent-snapshot-store.test.ts — save/load, directory auto-create, overwrite, loadAll (incl. skipping corrupt/unsupported/non-matching files), remove (incl. missing), and debounced scheduleSave + flush coalescing.
• server.test.ts — a created agent is written to disk, delete removes the file, and rehydrateAgents restores a persisted agent so it is served again.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.8 (1M context)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.26549% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.39%. Comparing base (7bd6550) to head (1fdca13).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
agent-service/src/server.ts	78.26%	15 Missing ⚠️
...nt-service/src/persistence/agent-snapshot-store.ts	90.90%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5276      +/-   ##
============================================
+ Coverage     49.01%   49.39%   +0.37%     
  Complexity     2378     2378              
============================================
  Files          1050     1051       +1     
  Lines         40336    40544     +208     
  Branches       4277     4277              
============================================
+ Hits          19772    20027     +255     
+ Misses        19407    19360      -47     
  Partials       1157     1157              

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)		Carriedforward from 7bd6550
agent-service	39.03% <90.26%> (+5.26%)	⬆️
amber	51.58% <ø> (ø)		Carriedforward from 7bd6550
computing-unit-managing-service	0.00% <ø> (ø)		Carriedforward from 7bd6550
config-service	0.00% <ø> (ø)		Carriedforward from 7bd6550
file-service	37.99% <ø> (ø)		Carriedforward from 7bd6550
frontend	40.82% <ø> (ø)		Carriedforward from 7bd6550
python	90.79% <ø> (ø)		Carriedforward from 7bd6550
workflow-compiling-service	56.81% <ø> (ø)		Carriedforward from 7bd6550

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