release-26.1: sql: add lookup fallback in internalLookupCtx

[fqazi]

Backport 1/1 commits from #164942 on behalf of @fqazi.

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Previously, when internalLookupCtx was used by crdb_internal or pg_catalog functions, it was assumed that reading system.namespace would cache all references in a database. While this worked when fetching descriptors from KV, this assumption can break when using leased descriptors.

This occurs because system.namespace may be newer than the leased descriptor timestamp. For example, if a table referenced by a foreign key is being dropped, the referencing table may still contain the entry. When the lookup context attempts to resolve that referenced table, the entry will be missing.

To address this, this patch allows internalLookupCtx to use a fallback, enabling it to query missing objects using the descriptor collection.

Fixes: #164430
Fixes: #164648

Release note (bug fix): Fixed a rare race condition where SHOW CREATE TABLE could fail with a "relation does not exist" error if a table referenced by a foreign key was being concurrently dropped.

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Release justification: low risk change that prevents errors with leased descriptors for crdb_internal / pg_catalog and concurrent drops

[blathers-crl]

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 

[rafiss]

pkg/sql/database.go:115:57: not enough arguments in call to newInternalLookupCtx
	have ([]"github.com/cockroachdb/cockroach/pkg/sql/catalog".Descriptor, "github.com/cockroachdb/cockroach/pkg/sql/catalog".DatabaseDescriptor)
	want ([]"github.com/cockroachdb/cockroach/pkg/sql/catalog".Descriptor, "github.com/cockroachdb/cockroach/pkg/sql/catalog".DatabaseDescriptor, internalLookupCtxFallbackFn)