impl(bigquery): add query method to client

[alvarowolfx]

Towards #5844

[gemini-code-assist]

Code Review

This pull request introduces a unified query method to the BigQuery client to prepare SQL query executions, and publicly exposes QueryReference. The review feedback suggests adding a doctest to the new public query method to comply with the repository's style guide on documenting public items.

[gemini-code-assist]

The repository style guide highly encourages adding doctests describing usage for all public items. Adding a doctest to the query method will improve usability and serve as a clear example for developers.

    /// Prepares a SQL query execution by returning a unified `RunQuery` request builder.
    /// This builder internally routes to either `jobs.query` (fast path) or `jobs.insert` (job path)
    /// depending on the fields configured.
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// # use google_cloud_bigquery::client::BigQuery;
    /// # #[tokio::main]
    /// # async fn main() -> Result<(), Box<dyn std::error::Error>> {
    /// let client = BigQuery::builder().build().await?;
    /// let query = client.query("SELECT 1")
    ///     .with_project_id("my-project")
    ///     .run()
    ///     .await?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn query<S: Into<String>>(&self, sql: S) -> RunQuery {
        RunQuery::new(self.job_service.clone(), sql.into())
    }

References

1. Documentation: Document all public items in crates that we publish with ///. Doctests describing usage are highly encouraged. ^(link)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.89%. Comparing base (75c4db2) to head (b8dbe7f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bigquery/src/client.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5926      +/-   ##
==========================================
- Coverage   97.90%   97.89%   -0.01%     
==========================================
  Files         234      234              
  Lines       59940    59943       +3     
==========================================
- Hits        58683    58682       -1     
- Misses       1257     1261       +4     

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