sql: sort rulesForRelease versions in descending order

This fix addresses incorrect ordering in the rulesForReleases array
and adds a runtime check to prevent future ordering errors.

Bug Summary:
Location: pkg/sql/schemachanger/scplan/plan.go:157-162

Issue: The rulesForReleases array has incorrect ordering. Entries V25_2
and V25_3 are in ascending order, but the documented requirement
(line 158) and the search logic in GetRulesRegistryForRelease() require
descending order (newest first).

Current (incorrect):
  {Latest},    // V25_4
  {V25_2},     // ← wrong position
  {V25_3},     // ← wrong position

Should be (descending):
  {Latest},    // V25_4
  {V25_3},     // ← correct
  {V25_2},     // ← correct

The GetRulesRegistryForRelease function iterates through rulesForReleases
in order and returns the first entry where activeVersion.IsActive() is
true. With ascending order, it would incorrectly return V25_2 rules when
V25_3 is active.

Changes:
- Fixed ordering in pkg/sql/schemachanger/scplan/plan.go
- Added init-time assertion to validate descending order
- Fixed trailing space in error message in pkg/cli/declarative_print_rules.go

Epic: None
Release note (bug fix): Fix rulesForReleases ordering to correctly match
cluster versions with schema changer rule sets.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: celiala

pkg/cli/declarative_print_rules.go

@@ -37,7 +37,7 @@ a given corpus file.
 					Version: version,
 				})
 			if rules == nil {
-				fmt.Printf("unsupported version number, the supported versions are: \n")
+				fmt.Printf("unsupported version number, the supported versions are:\n")
 				for _, v := range scplan.GetReleasesForRulesRegistries() {
 					fmt.Printf(" %s\n", v)
 				}