fix(message): correctly concat base64-encoded data block deltas

[qbc2016]

Description

DATA_BLOCK_DELTA was appended with naive string concatenation. When each delta is an independently base64-encoded chunk whose raw byte length isn't a multiple of 3, the per-chunk = padding gets spliced into the middle of the stream, corrupting the result on decode.

Fix: decode existing and incoming chunks, concat the raw bytes, and re-encode — so the accumulated value equals base64(concat(rawBytes)).

Test updated to use chunks that exercise mid-stream padding (AQI= + Aw== → AQID); the old string-concat path would have silently passed before.

Checklist

Please check the following items before the code is ready to be reviewed.

• This PR is linked to a related issue (see above)
• Code has been formatted with pnpm format
• Related documentation has been updated (e.g. links, examples, etc.)
• All tests are passing (pnpm test)
• Code is ready for review