fix(api): preserve optional ClickPipe create fields

[sdairs]

ClickPipeSettings.kafka_read_committed and ClickPipePostSource.validate_samples were modeled as required booleans, which forced callers to serialize false when those source-conditional create fields were unset. On 2026-09-05, a live object-storage create that omitted source.validateSamples succeeded; the same request with settings.kafka_read_committed:false failed with Setting 'kafka_read_committed' is only supported for Kafka ClickPipes, and omitting that setting succeeded.

This changes both request fields to Option<bool> with absent values omitted from JSON. Explicit Some(false) remains serialized. Narrow analyzer exemptions document the observed control-plane behavior, and model tests cover omitted and explicit-false payloads. The CLI receives compile-only Some(...) adaptations in this prerequisite commit, so its wire behavior remains unchanged until the follow-up CLI commit.

Validation:

• cargo fmt --all
• 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 -m unittest discover -s scripts/tests -p 'test_*.py' (91 passed with commit signing disabled for the temporary-repository test)
• cargo check --workspace --all-features

The prerequisite also adapts three CLI unit assertions to the new types. Its no-default all-target Clippy, affected unit tests and full CLI test compilation pass before the behavior-changing follow-up.

Prerequisite for #595.