Validate PostgreSQL ClickPipe inputs

[sdairs]

Summary

• validate PostgreSQL ClickPipe ports, required complete table mappings, IAM role authentication, and replication-slot mode before dispatch
• build PostgreSQL create payloads through a typed request builder with minimal, maximal, and mode-preservation coverage
• reject ignored --iam-role values under basic auth and document all conditional input rules

Tests

• cargo test -p clickhousectl
• cargo fmt --all --check
• cargo clippy -p clickhousectl --all-targets -- -D warnings

Closes #449

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 1a3a446. Configure here.}

[sdairs]

Carry-over review from PR #484 (old PR for #449). This PR strictly subsumes #484's fix and is approved on its own merits (uniform exit 2 before any auth lookup/network, per-mapping parse-time validation, explicit contract decision rejecting --iam-role under basic auth, README + after_help docs). No sdairs review comments existed on #484 to carry over. Two small items:

1. Trim nit (Cursor Bugbot finding on this PR, valid). parse_postgres_table_mapping_parts (crates/clickhousectl/src/cloud/clickpipes.rs:~1857-1869) validates emptiness via trim() but returns the untrimmed components, so " public.events:events " reaches the create payload padded. Either trim on return or reject padded components outright.

2. Decide/justify the ArgumentConflict error kind for the slot-vs-mode post-parse rule — it is not a true argument conflict, but it matches the established repo pattern (local postgres start uses it too), so this is likely fine as-is; just confirm intentional.