roachvet: add swallowed error detection analyzers

[ajstorm]

Summary

This PR adds 5 new static analysis passes to roachvet for detecting swallowed errors that could cause silent data loss or corruption:

• defererr: Detects errors in defer blocks that are logged but not propagated
• iteratorerr: Detects iterator loops where the error variable may not be checked after the loop exits
• errdelegate: Detects Err() methods that don't delegate to embedded types
• logerr: Detects log-and-swallow patterns
• txnerr: Detects swallowed errors in transaction closures

Current Findings

Running these analyzers on the codebase finds 10 issues (3 iteratorerr, 7 defererr).

Click to expand full findings

iteratorerr (3 issues) - Likely Real Bugs

These are cases where iterator loop errors are not checked after the loop exits:

File	Line	Issue
pkg/server/index_usage_stats.go	272	Iterator loop returns nil instead of err after loop
pkg/server/index_usage_stats.go	418	Iterator loop returns nil instead of err after loop
pkg/sql/scheduledlogging/captured_index_usage_stats.go	295	Iterator loop returns nil instead of err after loop

defererr (7 issues) - Mostly Intentional Patterns

These are cases where errors in defer blocks are logged but not propagated:

File	Line	Assessment
pkg/backup/backup_processor.go	442	Likely safe - progress reported after Finish() succeeds
pkg/ccl/cmdccl/clusterrepl/main.go	103	Intentional - best-effort cleanup on shutdown
pkg/ccl/sqlproxyccl/conn_migration.go	195	Intentional - recovery pattern with Close() fallback
pkg/cmd/dev/build.go	136	Dev tooling - less critical
pkg/cmd/dev/build.go	138	Dev tooling - less critical
pkg/cmd/roachprod-centralized/.../migrator.go	215	Roachprod tooling
pkg/roachprod/vm/gce/dns.go	469	Roachprod tooling

Test plan

• Unit tests for each analyzer in testdata/ directories
• Analyzers are integrated into roachvet and run via nogo during bazel builds

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🤖 Generated with Claude Code

Release note: none

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