fix: tighten iceberg metadata compatibility fallbacks

[EDsCODE]

Summary

• route simple, single-statement, and extended-protocol exec compatibility retries through the shared fallback helper
• suppress native Iceberg information_schema.columns duplicates explicitly when loaded metadata exists
• deduplicate ErrorResponse parsing test helpers and cover extended DROP SCHEMA CASCADE fallback

Tests

• go test ./server ./server/sessionmeta -count=1
• just test-unit
• git diff --check

Notes

• just lint was attempted locally but is blocked because golangci-lint is not installed in this environment