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 — 849 tests
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo +nightly fmt --check
• ./scripts/codegen.sh with no tracked-file drift; make uniffi reproduces the committed bindings byte-for-byte
• playground yarn test:unit, yarn lint; explorer npm run lint
• xcodebuild build-for-testing -scheme TrUAPIHost against a locally built simulator XCFramework

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.

The checked-in host-CLI reports still carry the older HostFailure Details strings for the nine CoinPayment methods; they refresh on the next CLI e2e run.

Open follow-ups, each a decision rather than effort:

• widening the native adapter past Text/Custom, or narrowing what the protocol advertises
• whether JS hosts serve Chat at all (A JS host cannot serve chat: executionKind: "Chat" is accepted, but every chat call returns Unsupported #383)
• a Kotlin chat bridge, which needs a TrUAPIHostRuntime wrapper first — TrUAPIHost.kt wraps only NativeTrUApiCore, and openProductExecution is the only entry point accepting chat callbacks
• error frames diverge from the reference JS host: CallError adds a tag and inner version that its ErrEnum does not expect, tracked as TODO(shared-core-wire) in frame.rs
• complete() is unreachable for error-channel subscriptions (ts.rs), which affects the live statement_store.subscribe today

[TarikGul]

Shared findings with @decrypto21 offline.