Add OumiConfigParsingError covering all OmegaConf exceptions

[idoudali]

Description

Introduce OumiConfigParsingError to wrap all OmegaConfBaseException subclasses (ValidationError, GrammarParseError, ConfigIndexError, etc.) so that every OmegaConf exception is reported as a clean user-facing error by the CLI instead of dumping a full traceback.

• exceptions.py: new OumiConfigParsingError with msg/full_key extraction
• base_config.py: 5 catch sites updated (from_yaml, from_str, from_yaml_and_arg_list)
• verl_grpo_trainer.py: catch site updated
• test_base_config.py: updated existing ConfigKeyError test, added GrammarParseError test
• test_config.py: two ValidationError tests now expect OumiConfigParsingError with cause chain

Fixes OPE-1852

Related issues

Fixes # (issue)

Before submitting

• This PR only changes documentation. (You can ignore the following checks in that case)
• Did you read the contributor guideline Pull Request guidelines?
• Did you link the issue(s) related to this PR in the section above?
• Did you add / update tests where needed?

Reviewers

At least one review from a member of oumi-ai/oumi-staff is required.

[Copilot]

Pull request overview

This PR introduces a dedicated OumiConfigParsingError to wrap OmegaConf parsing/validation exceptions so CLI/config loading surfaces clean, user-facing configuration errors (via OumiConfigError) rather than raw OmegaConf tracebacks.

Changes:

• Added OumiConfigParsingError (and OumiConfigTypeError) to the central exception hierarchy in src/oumi/exceptions.py.
• Updated BaseConfig.from_yaml(), from_str(), and from_yaml_and_arg_list() to catch OmegaConfBaseException and re-raise as OumiConfigParsingError with proper __cause__ chaining; replaced ad-hoc TypeError with OumiConfigTypeError.
• Updated/added unit tests to assert the new wrapper exception behavior and cause chaining (including a malformed interpolation case).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/unit/core/configs/test_config.py	Updates expectations to OumiConfigParsingError and asserts underlying OmegaConf cause is chained.
tests/unit/core/configs/test_base_config.py	Adds coverage for wrapped parsing errors (unknown field + malformed interpolation) and reuses a shared YAML fixture string.
src/oumi/exceptions.py	Adds OumiConfigParsingError (and OumiConfigTypeError) to support consistent, user-friendly config error handling.
src/oumi/core/configs/base_config.py	Wraps OmegaConf exceptions during YAML/string load and CLI arg merges into OumiConfigParsingError, and standardizes type mismatch errors.

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