feat(chat): implement Chat::register_bot in the shared Rust core

[decrypto21]

Summary

• serve chat.registerBot from the shared core through a ChatPlatform::register_bot hook, reaching native storage via NativeChatCallbacks and the Swift ChatHostBridge
• validate product-supplied chat ids, names and icons centrally on create_room and register_bot, and normalize post_message's room id to the same form
• answer CallError::Unsupported from all nine CoinPayment methods rather than inheriting the trait default's HostFailure
• pin the register_bot request bytes and status discriminants against the reference JS host's own codec
• derive the Chat diagnosis method list from the generated service metadata, run the playground unit tests in CI, and render a generated-but-unmeasured method as a gap in the explorer compatibility matrix
• record that a WASM/JS host cannot install a ChatPlatform, so executionKind: "Chat" there answers unsupported and its subscriptions end empty

register_bot reserves wire id 40 and is present in the generated dispatcher, the wire table and the published TS types, so the wire surface is unchanged by this PR — it converts a runtime unavailable into a real answer.

Chat coverage after this change

All six product-initiated Chat methods reach an implementation. The native adapter persists Text and Custom message content; the remaining five ChatMessageContent variants report a domain error, which a test pins so the ceiling is explicit rather than incidental. Worth naming: action_subscribe delivers ActionTriggered while Actions is one of the rejected variants, so a product can receive action triggers it cannot yet produce.

Field validation

create_room and register_bot bound their ids, names and icons, NFC-normalize them, and screen characters that let two distinct values render identically. Identifiers are screened harder than display names: a name keeps ZWJ and ZWNJ because emoji sequences and Persian need them, while an identifier rejects those plus variation selectors, soft hyphens, invisible operators and non-ASCII spaces. Icons are an allowlist — https, or an inline raster image — because a URL parser reaches a scheme through whitespace, tabs and NUL that a prefix comparison does not.

Contextual output escaping, storage limits, and every post_message field remain host-owned.

ChatHostBridge.registerBot is a required member

There is no protocol-extension default: a host either implements bot registration or fails to compile. A default that reported "unavailable" would land as an untyped Unknown{reason} — indistinguishable from a genuine failure, and the same additive-blind shape this change set exists to remove. StubChatHostBridge in the test target writes out every member, which is what makes the ios-swift job a gate.

Downstream needs one method. polkadot-app-ios-v2 has a live conformer on develop at polkadot-app/Modules/Products/TrUAPI/RustChatExecutionBridge.swift, implementing the four existing members. It needs a registerBot following the notImplemented(#function) pattern already in that file. The ios-swift job compiles only this repo's package, so CI here cannot see that break.

Validation

• cargo test --workspace --all-features — 914 tests
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo +nightly fmt --check
• cargo check --target wasm32-unknown-unknown -p truapi-server
• ./scripts/codegen.sh with no tracked-file drift; make uniffi reproduces the committed bindings byte-for-byte
• @parity/truapi 225 tests, @parity/truapi-host 49 tests; playground yarn test:unit, yarn lint, yarn build; explorer npm run lint

The Swift compile is covered by CI rather than locally: the TRUAPIProvider
product added in #276 resolves an unpublished @parity/ios-provider release
asset, so xcodebuild needs both TRUAPI_USE_LOCAL_BINARY=1 and
TRUAPI_PROVIDER_USE_LOCAL_BINARY=1 with both xcframeworks built first, which is
what the iOS package (swift compile) job does.

Each new assertion was mutation-checked against the defect it guards: removing the impl reproduces unavailable; swallowing a host rejection into a fabricated New fails; collapsing a field rejection into Unsupported fails; dropping post_message normalization orphans the room and fails; reordering bot_id/name fails the wire fixture; deleting a CoinPayment override restores HostFailure and fails; a spurious room-list republish fails rather than hanging.

Notes

Registering a bot does not republish the room list — list_rooms reports rooms, and a bot identity is not a room. A host that joins a bot to a room signals that through notify_chat_rooms_changed.

[TarikGul]

Shared findings with @decrypto21 offline.