UTS: fix publish-body assertions and pin protocol per test

[owenpearson]

Fixes the UTS test-spec defects in #527.

Defects fixed (contradictions with RSL4c3/RSL1k)

• channel/publish.md RSL1a publish-message-array-1: object payload was asserted unstringified; now asserts a stringified data string, decodes it, and asserts encoding == "json" (matching the message_encoding.md idiom).
• channel/publish.md RSL1e null-name-and-data-0: dropped the whole-body-equality against a literal that omits the idempotency id (RSL1k1); now asserts field-wise plus the existing NOT IN omission checks.
• channel/idempotency.md RSL1k mixed-ids-in-batch-1: removed the assertion that a library id is generated for the id-less message in a mixed batch (RSL1k1's precondition excludes this; RSL1k3 forbids it); now asserts "id" NOT IN body[1]. Prose rewritten to state RSL1k3.

Protocol pinned per test

useBinaryProtocol defaults to true (TO3f), so parse_json(request.body) only holds under JSON. Added useBinaryProtocol: false to the client in every section that reads the request body, across the enumerated specs.

Doc convention

Added a note to docs/writing-test-specs.md: unit specs default to JSON when unpinned, and any spec whose assertions depend on the encoding must pin useBinaryProtocol (citing message_encoding.md as the pattern).

Files changed

• uts/docs/writing-test-specs.md
• uts/rest/unit/channel/publish.md
• uts/rest/unit/channel/idempotency.md
• uts/rest/unit/channel/annotations.md
• uts/rest/unit/channel/update_delete_message.md
• uts/rest/unit/batch_publish.md (RSC22c6 pinned to JSON so the base64/encoding: "base64" branch is valid)
• uts/rest/unit/push/push_admin_publish.md
• uts/rest/unit/push/push_channels.md
• uts/rest/unit/push/push_channel_subscriptions.md
• uts/rest/unit/push/push_device_registrations.md

Merge order

PR 3 of 7. Merge after the mock-HTTP PR. Shares channel/idempotency.md with the spec-point-references PR and encoding/message_encoding.md with the broken-fixtures PR, so coordinate/rebase.

Closes #527

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