sql: add descriptors.json and schema_changes.txt to stmtbundle

[virajchogle]

When investigating optimizer surprises, the bundle reviewer often needs to rule out schema-state interactions. Two new files help: descriptors.json with the pretty-printed JSON for each descriptor used by the statement, and schema_changes.txt with the recent schema-change job history for those descriptors.

The declarative schema changer drains payload.DescriptorIDs as targets complete, so completed declarative jobs have an empty descriptorIds field. PrintSchemaChanges therefore also matches the descriptor's fully-qualified name against the job's description text.

Verified manually on a single-node demo cluster:

• ALTER TABLE ... ADD COLUMN and CREATE INDEX (declarative schema changer) both appear in schema_changes.txt with status, timestamp, and description.
• descriptors.json shows the table descriptor with the new column and secondary index.
• EXPLAIN ANALYZE (DEBUG, REDACT) replaces both files with the omitted-in-redacted-bundle placeholder and does not emit errors.txt.
• EXPLAIN ANALYZE (DEBUG) SELECT count(*) FROM pg_catalog.pg_class produces clean empty-state placeholders (no errors.txt).

Fixes: #170491
Epic: None
Release note (general change): Statement bundles produced by EXPLAIN ANALYZE (DEBUG) now include descriptors.json (pretty-printed descriptor JSON for each object used by the statement) and schema_changes.txt (recent schema-change history for those descriptors).

@michae2

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