Move ExplainMode analysis to access-mode analysis

[shirly121]

Description

Move the analysis of ExplainMode out of the physical analyzer and into access-mode analysis. An EXPLAIN query should be classified as read-only before execution starts, so that a ReadTransaction can be created before the query is prepared and executed.

Expected behavior

• EXPLAIN queries use a ReadTransaction.
• The transaction is created before query execution.
• Existing EXPLAIN and PROFILE behavior remains unchanged.

[github-actions]

🤖 Auto-triage: linked as sub-issue of #97 — Support EXPLAIN, PROFILE Queries (confidence: high).

Reason: The issue is about EXPLAIN query classification and ensuring EXPLAIN uses a ReadTransaction, which falls squarely under the EXPLAIN/PROFILE query introspection umbrella.

Auto-applied: type → Task; labels → compiler, executor

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