feat(chat): enhance message streaming logic for tool responses

[Gourav-InfoTech]

Description

Fixes a bug where post-tool text responses (e.g. after websearch/webanswer builtin tools) would either appear all-at-once instead of streaming live, or appear as duplicate messages when both the streaming path and the done handler inserted the same content.

Changes

• Detect message:delta chunks that arrive with no active streamState (server-side tools like websearch that skip message:start) and auto-initialize a new assistant message so post-tool text streams live
• Introduce postToolTextStreamed flag to signal that post-tool text was already delivered via the live-streaming path, preventing the done handler from inserting a duplicate
• Introduce seenToolResult tracking in the done.messages loop to correctly distinguish pre-tool assistant text (skip — already streamed) from post-tool assistant text (insert only if not already streamed)
• Refactor if/continue chain into a proper if/else if/else block for clarity

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Testing

• I've tested this locally
• I've added/updated tests
• All existing tests pass

Checklist

• My code follows the project's style guidelines
• I've updated the documentation (if needed)
• I've added tests that prove my fix/feature works
• New and existing tests pass locally

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