Explain the GCP scope of ClickPipe workload identity context

## Validated scope and current-PR plan — 2026-09-11

This section records the QA review and the current decision to finalize existing PRs without opening new PRs. It supersedes the proposed routing/fix suggestions in the original report below; the original observations are retained. Central plan: #757.

### Work for the existing PRs

Implementation status: the checked sub-items below are implemented on the existing PR branches and validated locally on combined head `82b374816a917cbac3a68194659beaaa5c7c9087`. Required final-head Cloud CI and merging remain pending; this issue stays open. See #757 for the combined validation and CI status.

- [x] #811 — Resolve item 6 (hidden empty fields) through the shared renderer; no separate ClickPipe-specific rendering change.

- [x] #814 — Resolve item 1: remove serde Option/untagged traversal artifacts from generic unknown-field paths and identify stdin input clearly. Retain strict unknown-field rejection and the specific BigQuery diagnostic.

### Remaining scope

GCP-context guidance, not-found rewrites (#831), delete-name output (#843) and JSON casing policy (#863) remain deferred.

This is a partial assignment. Keep the issue open after these PRs land; use sub-item references rather than a whole-issue closing keyword.

### Validation and corrections

Reproduced source.?.postgres.?.settings.?.initialLoadParallelism and “config -”. #814 is the direct missing follow-up: it special-cases BigQuery without fixing generic serde path rendering. Item 3 shares #831; item 4 shares #843. Item 2 is new GCP-context guidance. Item 5 belongs to #863’s contract discussion: snake_case settings are API wire names, not an accidental CLI rename. Item 6 duplicates #850.

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## Original QA report

Found in the 2026-09-11 QA sweep of the combined PR stack at [`d2c37f81`](https://github.com/ClickHouse/clickhousectl/commit/d2c37f814fff53fe68d636e34103e2d0550bf84d), top PR #823, installed release build (package version still 0.4.2). Reproduced behaviour only; no implementation included.

Severity: low-medium; batch of output defects in `clickpipes.rs`.

1. PATCH validation leaks `Option`/untagged-enum traversal into the field path:
`unknown field \`source.?.postgres.?.settings.?.initialLoadParallelism\``. Sibling messages are clean (`\`source.mysql\` is missing required field \`port\``). Also "in config -" when the body came from stdin. Strip `?` segments; say "config read from stdin".
2. `clickpipe context get` on any AWS service → `Error: BAD_REQUEST: GCP workload identity authentication is not available`, exit 1. The `about` ("Get service capabilities and workload identity") and the `create` CONTEXT block point users at it with no hint it is GCP-only. Scope the about or rewrite that 400.
3. `clickpipe delete "$SVC" "$GONE"` → `Error: NOT_FOUND: 536e7444-…` — a bare UUID. `get` on a well-formed unknown UUID → `Invalid clickpipe id. Expect a valid UUID, given: ` (doubled space, wrong claim). Rewrite to "ClickPipe not found".
4. `delete` prints the ID (`ClickPipe 536e7444-… deleted`) where `stop`/`start`/`resync` print the name; the delete response carries the name.
5. `clickpipe get/list/update/scale/cdc-scaling --json` emit camelCase (mirroring the API); `clickpipe settings get/update --json` emit snake_case (`object_storage_max_insert_bytes`). `get` also returns the same `settings` object in camelCase, so the same data has two spellings depending on the command.
6. Human `get` drops `fieldMappings: []`, `destination.table: ""`, `destination.columns: []` that JSON shows, while showing `settings: {}` (#806) — see #850.

The report points to clickpipes.rs and the clickpipe context get/about/create, delete, and get command paths. Start by reading the current validation and scope in #757, then check the remaining GCP-context item against the related work in #831, #843, and #863. Done means an explicitly assigned remaining item is validated without duplicating work already implemented in the existing PRs.

Written by the indexing model from the issue text.