Extract ClickPipes API models

[sdairs]

Parent: #317

Stack position: 5 of 7.

Goal

Extract the complete ClickPipes model surface into its own domain module as a move-only refactor.

Scope

• Move ClickPipe CRUD, source, scaling, settings, state, schema-discovery, credential, and reverse-private-endpoint models.
• Keep ClickPipes-owned leaves such as source credentials, PLAIN, MskIamUser, MutualTLS, AzureEventHub, and custom private DNS mappings in the ClickPipes module rather than a generic shared bucket.
• Move discriminated unions, response variants, aliases, enums, and associated implementations with their owning models.

Acceptance

• Existing public paths, request strictness, response tolerance, and wire names are unchanged.
• ClickPipes union and schema-discovery tests pass.
• ClickPipes integration-test binaries compile.
• Analyzer inventory and drift findings remain unchanged.
• Focused tests, clippy, rustfmt, and all-features workspace checking pass.

Out of scope

• ClickStack models.
• Any ClickPipes API or serialization behavior change.