fix(flow,task): handle pydantic outputs in guardrail retries and checkpoint serialization

[ibrahim1023]

Summary

Fixes #5544 by addressing two related issues when using Pydantic outputs in flows/tasks:

1. Guardrail retries could rebuild TaskOutput with a non-string raw value when the model returned a BaseModel.
2. Flow checkpoint serialization could fail with PydanticSerializationError when initial_state was assigned as a model class on flow subclasses.

What Changed

• Unified TaskOutput construction in lib/crewai/src/crewai/task.py so initial execution and guardrail retry paths use the same output-shaping logic.
• Updated sync and async guardrail retry code paths to use that shared builder, preserving expected raw/pydantic semantics for structured outputs.
• Ensured Flow.initial_state remains excluded from serialized flow entities and preserved inherited field metadata when flow subclasses override parent fields in FlowMeta (lib/crewai/src/crewai/flow/flow.py).
• Added focused regression tests:
  • test_guardrail_retry_preserves_pydantic_output (lib/crewai/tests/test_task.py)
  • test_ainvoke_guardrail_retry_preserves_pydantic_output (lib/crewai/tests/task/test_async_task.py)
  • test_flow_checkpoint_handles_nested_pydantic_models (lib/crewai/tests/test_checkpoint.py)

Why

• Guardrail retries should behave identically to first-pass execution for typed outputs.
• Checkpoint serialization should snapshot runtime state, not class-valued configuration artifacts.

Validation

Passed targeted suites for the touched behavior:

• uv run pytest lib/crewai/tests/test_checkpoint.py -q
• uv run pytest lib/crewai/tests/test_task.py -q -k 'guardrail or output_pydantic'
• uv run pytest lib/crewai/tests/task/test_async_task.py -q -k 'guardrail'

Also confirmed new regression tests pass individually after formatting.

Notes

• Full repo CI-equivalent runs in this local environment show broad unrelated failures due to missing optional dependencies/providers (for example a2a, provider SDK extras, qdrant, oxylabs) and interactive tool prompts in some tool tests.
• No unrelated code changes are included in this patch scope.