fix(chat): complete conversationId handling for assistant SSE

[vitorhugo-dotnet]

Summary

• validate conversationId before dispatching the assistant SSE request
• regenerate invalid persisted conversation IDs while preserving local message history
• surface HTTP/request failures separately from streamed provider failures
• make the frontend mock enforce the same conversationId contract as the backend
• cover first/subsequent messages, invalid persisted conversation state, and HTTP validation failures

Verification

• TDD RED confirmed the two missing behaviors before the production fix
• Playwright: 76 passed
• Production build: passed (tsc -b && vite build)
• Final diff reviewed: 5 files, scoped to assistant conversation handling/tests

Fixes #116