refactor(workflow): move AgentNode events out of graphon

[laipz8200]

Summary

• Move AgentNode-specific node and graph events into core.workflow.nodes.agent.
• Let AgentNode convert its local log event while delegating generic events to graphon.
• Update workflow streaming consumers to use the local graph event.

Fixes #39844

Screenshots

Not applicable; backend-only refactor.

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	56.86%	56.86%	+0.00%
Strict coverage	56.38%	56.38%	+0.00%
Typed symbols	37,167	37,168	+1
Untyped symbols	28,442	28,442	0
Modules	3092	3094	+2

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.18%. Comparing base (c562f78) to head (9828fa7).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #39845   +/-   ##
=======================================
  Coverage   86.18%   86.18%           
=======================================
  Files        5086     5087    +1     
  Lines      287694   287732   +38     
  Branches    57356    57356           
=======================================
+ Hits       247950   247990   +40     
+ Misses      34941    34939    -2     
  Partials     4803     4803           

Flag	Coverage Δ
api	86.37% <100.00%> (+<0.01%)	⬆️

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