[rllib] Improve invalid input error messages

[AksodFlare]

Description

This PR replaces tuple-style ValueError construction in RLlib offline IO and
serialization helpers with clear string messages, and adds regression tests for
the updated error text.

Related issues

N/A

Additional information

Affected areas:

• RLlib offline dataset writer
• RLlib filter error handling
• RLlib space serialization helpers

Testing

• python -m py_compile rllib/offline/dataset_writer.py rllib/offline/tests/test_dataset_writer.py rllib/utils/filter.py rllib/utils/tests/test_filter.py rllib/utils/serialization.py rllib/utils/tests/test_serialization.py
• pytest not run locally because gymnasium is missing in the current environment

[gemini-code-assist]

Code Review

This pull request updates several ValueError messages across the codebase to use f-strings for better readability and formatting. Additionally, it introduces new unit tests to verify these error messages. A high-severity issue was identified in the new test file rllib/offline/tests/test_dataset_writer.py, where the _DummyDataset mock class lacks a required repartition method, which would cause the test to fail with an AttributeError.

[gemini-code-assist]

The _DummyDataset class is missing the repartition method, which is called on the object returned by data.from_items in DatasetWriter.write (line 74). Without this method, the test will fail with an AttributeError instead of verifying the expected ValueError message.

Suggested change

	class _DummyDataset:
	def write_json(self, *args, **kwargs):
	pass
	def write_parquet(self, *args, **kwargs):
	pass
	class _DummyDataset:
	def repartition(self, *args, **kwargs):
	return self
	def write_json(self, *args, **kwargs):
	pass
	def write_parquet(self, *args, **kwargs):
	pass

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

[pseudo-rnd-thoughts]

Thanks for the PR @AksodFlare, looks good.