Add Neon DB sandbox preflight probes

[never2average]

Summary

• add generic Postgres and Neon readiness probes for bottom-up DB sandbox validation
• route the aggregate DB sandbox probe by source provider, including Neon and Supabase-style Postgres URLs
• add Ardent BYOC Neon connector creation support with API-key redaction and document the known snapshot_max_connections failure path
• make the AWS billing probe date logic portable on macOS

Tests

• bash -n scripts/probes/check-postgres-source-readiness.sh scripts/probes/check-neon-postgres-readiness.sh scripts/probes/check-db-sandbox-readiness.sh scripts/probes/check-ardent-connector.sh scripts/probes/check-aws-billing.sh
• scripts/probes/check-postgres-source-readiness.sh --dry-run
• scripts/probes/check-neon-postgres-readiness.sh --dry-run
• THINWEDGE_DB_SOURCE_PROVIDER=neon scripts/probes/check-db-sandbox-readiness.sh --dry-run
• scripts/probes/check-ardent-connector.sh --create --source-provider neon --neon-project-id twilight-lab-63846303 --dry-run
• cargo fmt --check -p thinwedge-cli
• CARGO_TARGET_DIR=/Users/priyeshsrivastava/codex/thinwedge-rs/target cargo test -p thinwedge-cli ardent_cmd

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