Extract ClickPipes API models

[sdairs]

Closes #381

Summary

• Extract the full ClickPipes CRUD, source, scaling, settings, state, schema-discovery, credential, and reverse-private-endpoint model surface.
• Keep ClickPipes-owned credential and private-DNS leaves in the ClickPipes domain module.
• Preserve all 221 relocated declarations/impls, public paths, Serde contracts, and analyzer inventory exactly.
• Compile every ClickPipes integration target and add representative facade path coverage.

Tests

• cargo fmt --all --check
• cargo check --workspace --all-features
• cargo test -p clickhouse-cloud-api -p clickhouse-openapi-analyzer
• cargo clippy -p clickhouse-cloud-api -p clickhouse-openapi-analyzer --all-targets -- -D warnings

[Copilot]

Pull request overview

This PR refactors the clickhouse-cloud-api crate by extracting the full ClickPipes model surface into a dedicated domain module while preserving the existing public API surface (api::… and api::models::…) and Serde/analyzer contracts.

Changes:

• Added a new src/models/clickpipes.rs module containing the ClickPipes CRUD/source/scaling/settings/state/schema-discovery/credential/reverse-private-endpoint models.
• Extended the model facade test to assert that key relocated ClickPipes types remain accessible via both root and models paths.

Reviewed changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/clickhouse-cloud-api/tests/model_facade_test.rs	Adds facade path/type-equality assertions for representative ClickPipes models.
crates/clickhouse-cloud-api/src/models/clickpipes.rs	Introduces the extracted ClickPipes model surface in a dedicated models submodule.

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[iskakaushik]

LGTM