Absorb create-time visible_to_clients on CreateTool (BC3 #12386 merged)

[jeremy]

Summary

spec/api-gaps/dock-tool-visible-to-clients.md records that BC3 #12386 ("Honor create-time visible_to_clients on dock tool creates", bee714c74, merged 2026-07-23) added an optional top-level visible_to_clients boolean to the dock-tool create endpoint (POST /buckets/:bucket_id/dock/tools.json, modeled as CreateTool). The entry was registered but not absorbed.

Contract (narrower than the six content creates)

• The flag is honored only for tool types that manage their own client visibility — Chat::Transcript and Kanban::Board, which otherwise start hidden. Pass true to create one already client-visible.
• All other tool types ignore it and inherit the project default.
• It applies only at create time; re-enabling an existing tool keeps its current visibility.

Absorption (this issue's PR)

Part of the post-#401 follow-up program (PR-1).

• Smithy: add visible_to_clients: Boolean (tri-state) to CreateToolInput, documenting the two-tool-type honoring rule.
• Go: CreateToolOptions.VisibleToClients *bool + tri-state pass-through in ToolsService.Create.
• Tests: Go tri-state transport test + one body-encoding test in each of TS/Ruby/Python/Kotlin/Swift asserting explicit false reaches the wire.
• Flip the gap entry to absorbed-in-sdk with the smithy_ref; update the api-gaps README row.

Coordination record for the registry scheme; the code ships in the linked PR.