Cleanup example metadata parsing utilities(#20251)

[cj-zhukov]

Which issue does this PR close?

• Closes #Cleanup example metadata parsing utilities #20251.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[cj-zhukov]

High-level overview

This PR follows up on review feedback and focuses on small structural and correctness improvements, without changing any user-facing behavior.

Specifically, this PR:

• Moves the ABBREVIATIONS constant from render to model to eliminate an unnecessary circular dependency between modules.
• Improves discover_example_groups to better handle edge cases, such as empty example roots or directories named main.rs. The function now fails fast with a clear error when no valid example groups are found.
• Adds additional unit tests for the discovery logic to cover these edge cases and make the behavior explicit and well-documented.

[cj-zhukov]

@martin-g I’ve addressed your comments in this PR by refactoring the module structure, fixing the example group discovery edge cases, and adding tests to cover them.
Would appreciate another look when you have time - happy to iterate further on naming or structure if needed.

[cj-zhukov]

@comphead @Jefffrey since you helped me with previous PRs related to example docs generation, it would be great if you could take a look at this one as well. Your feedback or any improvements would be much appreciated.

[comphead]

Suggested change

	return Err(DataFusionError::Execution(format!(
	return exec_err!(

[cj-zhukov]

Good point! I'm going to update to use exec_err! for consistency with the rest of code.

[cj-zhukov]

I updated the code to use exec_err! in places where it fits naturally, did a small refactor without changing behavior, and aligned imports with the conventions used elsewhere in datafusion-examples utilities

[comphead]

Thanks @cj-zhukov and @Jefffrey