refactor: centralize client tool result construction

[renal128]

Summary

• Replace the string and Encodable sendToolResult overloads with one method accepting ClientToolResultEvent.
• Move JSON encoding into a throwing ClientToolResultEvent initializer while preserving verbatim string construction.
• Update SDK examples and tool-result tests to use the event-based API.

Test plan

• swiftformat 'Sources' 'Tests' --lint --strict
• Focused startup/client/audio/tool tests
• swift test

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Note

Medium Risk
Breaking public API for tool results and callbacks; behavior is straightforward but every integrator must update call sites. No auth or connection-path changes.

Overview
sendToolResult on Conversation and ConversationClient now takes a single ClientToolResultEvent instead of separate for / result / isError overloads. Callers build results with ClientToolResultEvent.init(...) — string payloads stay verbatim; Encodable values are JSON-encoded via a new throwing initializer on the event type.

The client-tool callback is renamed from onUnhandledClientToolCall to onClientToolCall, and incoming clientToolCall events invoke that callback (not only “unhandled” cases). README, Usage docs, and tests follow the event-based pattern.

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There are 2 total unresolved issues (including 1 from previous review).

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