fix: propagate default target column warnings

[Saloni-0465]

Reference Issues/PRs

N/A

What does this implement/fix? Explain your changes.

This PR ensures that warnings generated during data conversion are surfaced in the top-level validation output.

Previously, if target_column was not provided, to_sktime_format would default to using the first column and store a warning inside the adapter metadata. However, this warning was not included in validation["warnings"], which is what downstream users (including agents) are expected to check.

With this change, any warnings generated during conversion are now propagated to the top-level validation report for both synchronous and asynchronous data loading. This makes the behavior more transparent and easier to debug.

Does your contribution introduce a new dependency? If yes, which one?

No.

What should a reviewer concentrate their feedback on?

• Whether metadata-level conversion warnings should be merged into top-level validation
• Whether the helper location in executor.py is appropriate
• Test coverage for sync and async loading

Any other comments?

Added regression tests for sync and async load_data_source warning propagation.

Validation:

• python -m compileall src/sktime_mcp/runtime/executor.py tests/test_data_validation_warnings.py
• Manual sync and async checks confirm the default-target warning appears in validation["warnings"]

Note: local pytest exits with code -1 before output in this environment, so I verified behavior directly.

PR checklist

For all contributions

• I've added myself to the list of contributors.
• Optionally, I've updated CODEOWNERS.
• I've added unit tests and made sure they pass locally where possible.

For new estimators

• Not applicable