Add placeholders to plugin string settings

[pedroapfilho]

What was wrong

Plugin setting descriptors had no placeholder field. The host rejects unknown descriptor keys, so plugins such as Tokenmaxx could not provide input guidance in host-rendered settings forms. Unset secret inputs always showed the generic [not set] marker.

What changed

• Added placeholder to string setting descriptors and validated it as a non-empty optional string at the plugin host and server contract boundaries.
• Rendered plugin-provided placeholders in desktop and mobile forms. Unset secrets use the declared placeholder, while configured secrets keep the [set] marker.

How you verified

• Added fail-before, pass-after coverage for descriptor validation, contract transport, regular string inputs, unset secrets, and configured secrets.
• pnpm exec turbo run test --filter=@bb/app --force: 3,157 passed, 3 skipped.
• pnpm exec turbo run test --filter=@get-bb/plugin-sdk --force: 128 passed.
• Turbo typechecks passed for @bb/mobile, @get-bb/plugin-sdk, and @bb/server-contract.
• ESLint passed on every changed TypeScript and TSX file.

AGENT GENERATED: by GPT-5