Stream live AI responses letter-by-letter instead of word-by-word

[MelvinBot]

Problem

Currently, live AI responses in chat are streamed word-by-word. This creates a noticeably chunkier experience compared to suggested responses, which stream smoothly letter-by-letter.

Expected Behavior

Live AI responses should stream letter-by-letter (character-by-character), matching the smooth streaming behavior already used for suggested responses. This provides a more polished, real-time feel that users expect from modern AI chat interfaces.

Acceptance Criteria

• Live AI responses render incrementally character-by-character, not word-by-word
• The streaming cadence visually matches the smoothness of suggested response streaming
• No performance regressions from the finer-grained rendering updates

[inimaga]

On hold first until the following PRs get merged and deployed:

• https://github.com/Expensify/Auth/pull/21452
• [Payment due @ChavdaSachin] Strip incomplete markdown from Concierge streaming drafts #89744

[inimaga]

Following disabling of streaming here, on hold till Streaming gets re-enabled for Expensifail domains via explicit betas through this PR

[inimaga]

Now coming off hold.