Fix MessageValidator default message metadata

[fallintoplace]

Summary

• Generate message IDs and timestamps inside the MessageValidator factory methods when callers omit them.
• Replace factory-level content defaults with None where the helper builds fallback content.
• Add regression coverage for all four WebSocket message factories.

Root cause

Python evaluates default arguments once when the function is defined. The previous UUID, timestamp, and content defaults were reused across calls that omitted those parameters, which could duplicate WebSocket message IDs and preserve stale timestamps in validation/error paths.

Validation

• uv run --frozen --extra test pytest packages/nvidia_nat_core/tests/nat/front_ends/test_message_validator.py
• uv run --frozen --extra test pytest packages/nvidia_nat_core/tests/nat/server/test_unified_api_server.py -k "websocket_message or create_observability_trace_message or websocket_error_message"
• uv run --frozen ruff check packages/nvidia_nat_core/src/nat/front_ends/fastapi/message_validator.py packages/nvidia_nat_core/tests/nat/front_ends/test_message_validator.py

Summary by CodeRabbit

New Features

• Enhanced message creation capabilities with support for optional custom identifiers and timestamps; system automatically generates these values when not explicitly supplied.
• Improved system message content defaults with optional configuration.

Tests

• Comprehensive test coverage added to verify automatic ID and timestamp generation across all message creation scenarios.

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ℹ️ Recent review info

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Run ID: 1cadda0b-d083-4a31-b4a3-b3daa6cce0a2

📥 Commits

Reviewing files that changed from the base of the PR and between 1cd7a85 and 12b0564.

📒 Files selected for processing (2)

• packages/nvidia_nat_core/src/nat/front_ends/fastapi/message_validator.py
• packages/nvidia_nat_core/tests/nat/front_ends/test_message_validator.py

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Walkthrough

Four factory methods in MessageValidator are refactored to accept optional message_id, content, and timestamp parameters, moving default value generation from function signatures to method bodies. Tests validate that defaults are correctly generated when parameters are omitted.

Changes

Message Factory Default Refactoring

Layer / File(s)	Summary
Factory method signature and implementation updates packages/nvidia_nat_core/src/nat/front_ends/fastapi/message_validator.py	create_system_response_token_message, create_system_intermediate_step_message, create_system_interaction_message, and create_observability_trace_message now accept optional message_id and timestamp parameters. Default value generation for UUIDs and timestamps moves from function signatures into method bodies. SystemResponseContent and SystemIntermediateStepContent become optional parameters with defaults assigned inside the method.
Test validation of factory defaults packages/nvidia_nat_core/tests/nat/front_ends/test_message_validator.py	Imports are expanded to include datetime, uuid, and additional message type dependencies. New TestMessageFactoryDefaults test class monkeypatches UUID and datetime generation inside the message validator module, then invokes all four factory methods to verify that fresh sequential IDs and incrementing timestamps are generated when optional parameters are omitted.

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change—fixing how MessageValidator handles default message metadata by generating them inside factory methods rather than at definition time.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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