docs(bigquery): improve Query handles docs

[alvarowolfx]

Towards #5844

[gemini-code-assist]

Code Review

This pull request adds comprehensive documentation and usage examples to the Query, CompleteQuery, and RunQuery structs in the BigQuery query module. The feedback points out incorrect import paths for QueryReference in the newly added documentation and examples, which would cause broken intra-doc links and rustdoc warnings.

[gemini-code-assist]

The QueryReference type is imported from crate::query::QueryReference (defined in src/bigquery/src/query/query_reference.rs), not crate::model::QueryReference. Using the incorrect path will result in broken intra-doc links and rustdoc warnings, which can fail the CI build.

Suggested change

	/// Returns the [`QueryReference`](crate::model::QueryReference) identifying this query execution.
	///
	/// The reference will be [`QueryReference::Job`] with a query [job reference],
	/// or [`QueryReference::Stateless`] with an opaque query ID if job creation
	/// was skipped.
	/// The reference will be [`QueryReference::Job`](crate::model::QueryReference::Job) containing a BigQuery Query [job reference]
	/// if a job was created, or [`QueryReference::Stateless`](crate::model::QueryReference::Stateless) with an opaque
	/// query ID if the execution ran statelessly via [jobs.query].
	/// Returns the [`QueryReference`](crate::query::QueryReference) identifying this query execution.
	///
	/// The reference will be [`QueryReference::Job`](crate::query::QueryReference::Job) containing a BigQuery Query [job reference]
	/// if a job was created, or [`QueryReference::Stateless`](crate::query::QueryReference::Stateless) with an opaque
	/// query ID if the execution ran statelessly via [jobs.query].

References

1. Always verify that Rust documentation compiles without warnings (e.g., by running cargo doc) before merging, because rustdoc warnings (such as bare URLs or unresolved links) will fail CI builds that enforce -D warnings.

[gemini-code-assist]

Update the import path in the example to use query instead of model to match where QueryReference is exported.

Suggested change

	/// use google_cloud_bigquery::model::QueryReference;
	/// use google_cloud_bigquery::query::QueryReference;

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.06%. Comparing base (210fed0) to head (32e92a7).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6212   +/-   ##
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  Coverage   96.06%   96.06%           
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  Files         268      268           
  Lines       67175    67175           
=======================================
  Hits        64534    64534           
  Misses       2641     2641           

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