fix(api): preserve partial ClickPipe PATCH fields

[sdairs]

Corrects the ClickPipe PATCH request models so fields proven optional can be omitted instead of forcing callers to fabricate credentials, settings, arrays, or validateSamples: false. Kafka credentials and reverse private endpoint IDs, Postgres credentials/settings/add/remove mappings, and source validateSamples now use omission-preserving Option fields. Destination columns remains required because the live API rejects an included empty destination object.

The narrow analyzer exemptions are grounded by a live object-storage PATCH on 2026-09-05 and the official Terraform provider's released Kafka update path and merged mappings-only fix (ClickHouse/terraform-provider-clickhouse#618). Model tests distinguish omission from explicit false and empty arrays, including Kafka CA-only changes and partial Postgres changes.

Validation:

• cargo fmt --all --check
• cargo clippy -p clickhouse-cloud-api -p clickhouse-openapi-analyzer --all-targets -- -D warnings
• cargo test -p clickhouse-cloud-api -p clickhouse-openapi-analyzer
• python3 scripts/check-openapi-drift.py --dry-run — 0 actionable drift
• Python script tests: 90 passed in the aggregate; the single sandbox-blocked temporary Git commit fixture passed on its exact unsandboxed rerun

Prerequisite for #569.