feat: enforce output_model validation in EventLoopNode (fixes #5929)

[Waryjustice]

Summary

Closes #5929.

NodeSpec.output_model and max_validation_retries were defined and tested, but never read by the runtime. A node declaring output_model=OrderSchema behaved identically to one without it, silently passing malformed LLM outputs into shared memory.

This PR wires output_model into the ACCEPT path of EventLoopNode.execute().

Changes

\core/framework/graph/event_loop_node.py\

• Import \OutputValidator\ from \ramework.graph.validator\
• Initialise _validation_retry_count = 0\ alongside the other loop counters
• After the missing-keys guard in the ACCEPT path, call \OutputValidator.validate_with_pydantic(accumulator.to_dict(), output_model):
  • ValidationError + retries remaining → inject [Output validation failed]: \ into conversation history and \continue\ the loop (RETRY)
  • ValidationError + retries exhausted → ESCALATE with a structured error message, log via
    untime_logger, and return a failed \NodeResult\
  • Validation passes → fall through to the existing memory-write path unchanged

\core/tests/test_pydantic_validation.py\

• New \TestRuntimeValidationPath\ class with 8 tests that mirror the exact validation logic in the ACCEPT path:
  • Valid outputs pass silently
  • Invalid outputs produce properly-formatted feedback
  • Injected message matches the [Output validation failed]:\ prefix
  • Default retry budget (2) is tracked correctly
  • \max_validation_retries=0\ escalates on the first failure
  • Nodes without \output_model\ skip validation (zero behaviour change)
  • Type-coercion failures and constraint violations are caught
  • Successful validation returns the correct model instance

Behaviour unchanged for existing nodes

Nodes that do not set \output_model\ take the \if ctx.node_spec.output_model is not None\ fast path and are completely unaffected.

[github-actions]

PR Closed - Requirements Not Met

This PR has been automatically closed because it doesn't meet the requirements.

PR Author: @Waryjustice
Found issues: #5929 (assignees: none)
Problem: The PR author must be assigned to the linked issue.

To fix:

1. Assign yourself (@Waryjustice) to one of the linked issues
2. Re-open this PR

Exception: To bypass this requirement, you can:

• Add the micro-fix label or include micro-fix in your PR title for trivial fixes
• Add the documentation label or include doc/docs in your PR title for documentation changes

Micro-fix requirements (must meet ALL):

Qualifies	Disqualifies
< 20 lines changed	Any functional bug fix
Typos & Documentation & Linting	Refactoring for "clean code"
No logic/API/DB changes	New features (even tiny ones)

Why is this required? See #472 for details.