release-26.2: stmtdiagnostics: add continuous bundle collection with COUNT-based limiting

[dhartunian]

Backport 3/3 commits from #162154 on behalf of @isaactwong.

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Summary

• Adds request_id column and index to system.statement_diagnostics via schema migration (V26_2_StmtDiagnosticsRequestID), linking bundles to their originating diagnostic request.
• Implements continuous diagnostic bundle collection for requests with sampling_probability and expires_at set. At insertion time, enforces per-request limits by counting existing bundles (SELECT COUNT(*) WHERE request_id = ?) against the sql.stmt_diagnostics.max_bundles_per_request cluster setting (default 10).
• During mixed-version upgrades, continuous requests collect without limit enforcement until the migration completes. Pre-migration bundles have NULL request_id, so the counter resets to 0 post-migration.

Commit breakdown

1. stmtdiagnostics: add request_id schema migration — Version gate, schema change, migration, golden file updates.
2. stmtdiagnostics: add continuous bundle collection with COUNT-based limiting — Registry implementation, cluster setting, tests.

🤖 Generated with Claude Code

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Release justification: missed the branch cut yesterday due to merge conflicts

[blathers-crl]

⚠️ End-of-Life Version Warning

This backport targets 26.2, which is an End-of-Life (EOL) version.

Please verify that backporting to this EOL version is intentional and appropriate. EOL versions no longer receive maintenance updates according to our support policy.

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Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is 

[themavik]

What it does: Adds continuous statement diagnostic bundle collection with COUNT-based per-request limiting, plus request_id schema migration.

Done well: Clear migration path (V26_2_StmtDiagnosticsRequestID) with version gating. The mixed-version behavior (no limit until migration completes, NULL request_id for pre-migration bundles) is sensible. Good test coverage in statement_diagnostics_test.go. Cluster setting sql.stmt_diagnostics.max_bundles_per_request gives operators control.

Suggestion: The SELECT COUNT(*) at insertion time could become a hotspot under heavy continuous collection. Consider whether a cached counter or batched cleanup would help at scale—or document that operators should tune max_bundles_per_request for high-throughput scenarios.