Add a Query option for ShowQuery instead of requiring query_stats.query_text

[apstndb]

Context

In-process graph rendering wants --show-query (or equivalent) to print the SQL that the caller already holds. visualize.Plan and the dot / graphviz / mermaid / d2 renderers only build the query-text node from ResultSetStats.query_stats key query_text:

// visualize/build_tree.go (v0.11.0)
const queryTextKey = "query_text"
q := &graphmodel.QueryText{Text: m[queryTextKey].GetStringValue()}

There is no Query string field on visualize.BuildOptions, visualize.GraphOptions, or the renderer option structs. The CLI path is fine because Spanner PROFILE JSON usually already contains query_text. Library callers that executed the statement themselves typically have the SQL as a separate string and an empty or stats-only QueryStats map.

execspansql therefore clones ResultSetStats and injects the key before visualize.BuildPlan:

https://github.com/apstndb/execspansql/blob/ff4f7d15df6a88d8d4c9d9ba57e636fcc0c54bb7/internal/planrender/render.go#L213-L225

cloned.QueryStats.Fields["query_text"] = structpb.NewStringValue(opts.Query)

That works, but it mutates a cloned protobuf using an undocumented map key and is easy to get wrong (missing clone, overwriting other stats, ShowQuery true with empty text).

Verified against published github.com/apstndb/spannerplanviz@v0.11.0 and current main (visualize/graph.go, mermaid/options.go). No existing issue.

Request

Add a first-class Query string on the renderer / graph options (or on visualize.BuildOptions / Plan) that ShowQuery uses when set.

If a dedicated field is not wanted, document query_stats.query_text as the supported contract for ShowQuery on BuildPlan / BuildGraph so in-process callers can rely on it without reading buildQueryText.

Alternatives considered

• Keep cloning ResultSetStats in every downstream (current execspansql workaround).
• Require callers to round-trip through JSON that already contains query_text (the CLI shape). That is unnecessary when the SQL is already in memory.
• Put the string only on mermaid/d2 Options and not on dot/graphviz. Worse: ShowQuery already exists on every backend.

Preferred: explicit Query string that takes precedence over query_stats.query_text, leaving the map key as the file/CLI interchange.

Downstream evidence

execspansql PRs #94 (in-process internal/planrender over spannerplanviz v0.11.0) and #95 (plan artifact sink that later feeds that renderer). Merged at ff4f7d1.