fix: join text blocks with empty string in finalText

[frankgoldfish]

Summary

• Fixes #getFinalText() in src/lib/MessageStream.ts to join multiple text blocks with an empty string instead of a space

Bug

#getFinalText() was calling textBlocks.join(' '), which inserts an unexpected space character between text content blocks:

// Before (buggy)
return textBlocks.join(' ');   // "Hello world" with an extra space

When a response contains two adjacent text blocks ("Hello" and " world"), the result would be "Hello world" (double space) instead of "Hello world".

Fix

// After (correct)
return textBlocks.join('');

This matches the Python SDK which uses "".join(text_blocks) (empty separator), and the docstring which says the blocks are concatenated (not space-joined).

Test plan

• Existing base64/MessageStream tests still pass
• Consistent with Python SDK behaviour ("".join(text_blocks))
• Matches the JSDoc description: "concatenated together if there are more than one text blocks"

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