feat: add DistributedQueryExec::explain_executed_plan and show logical plan in display

[andygrove]

Which issue does this PR close?

No tracking issue was filed for this change. Happy to open one if preferred.

Rationale for this change

After running a distributed query, there is no convenient way for a client to see the physical plan that actually executed on the cluster. DistributedQueryExec only holds the logical plan, and until now its display showed a single scheduler_url=... line. The distributed physical plan (the per-stage DAG with shuffle boundaries) is built and retained on the scheduler and was only surfaced through EXPLAIN ANALYZE, always bundled with runtime metrics. This PR lets a client fetch that executed plan on demand (with or without metrics) and makes the operator's default display more informative.

What changes are included in this PR?

• Add DistributedQueryExec::explain_executed_plan(&self, session_config, with_metrics), which fetches the executed per-stage plan for an already-run query via the existing get_job_metrics gRPC and returns it as a printable string. with_metrics = true matches the EXPLAIN ANALYZE output; false omits the metrics=[...] suffixes. It returns an error if called before the query has executed.
• Extract the per-stage plan formatting out of the explain-analyze RecordBatch builder into a reusable format_job_plan(job_metrics, with_metrics), and widen fetch_job_metrics to pub(crate). Existing EXPLAIN ANALYZE output is unchanged (the record-batch builder now delegates to format_job_plan(.., true)).
• Extend DistributedQueryExec's DisplayAs (Default/Verbose) to also render the submitted logical plan via LogicalPlan::display_indent(). This is synchronous and makes no scheduler round-trip; the TreeRender arm is unchanged.

No proto, planner, or scheduler changes. Covered by unit tests (formatter without metrics, display includes logical plan, error-before-execution) and an end-to-end integration test in context_checks.rs that runs a query and asserts the executed plan renders with and without metrics.

Are there any user-facing changes?

Yes, additive and non-breaking:

• New public method DistributedQueryExec::explain_executed_plan.
• DistributedQueryExec's default display now includes the submitted logical plan in addition to the scheduler URL.

No breaking changes to public APIs.

[andygrove]

@timsaucer I'm still working on this, but fyi

[milenkovicm]

Thanks @andygrove