[dsilvadeepal]

Fixes #22221.

IOHandler.deserialize_streaming_output uses character-set stripping to remove the JSON envelope. This drops valid leading letters and trailing brackets/quotes; for example, data engineering becomes engineering.

Use exact prefix/suffix removal while retaining the existing fragment decoding behavior. Add regression coverage for full responses and fragments, including a mocked SageMakerLLM.stream_complete test that verifies cumulative text and deltas. No dependency changes.

Validation against main 619d746:

• Before fix: 15 failed, 6 passed.
• After fix: all 21 package tests passed; package coverage 79%.
• Ruff lint/format and git diff --check passed.
• Tests use synthetic payloads and a mocked AWS client; no live AWS validation.

This does not add buffering for arbitrary byte splits inside JSON escapes, UTF-8 characters, or the envelope.

Related earlier unmerged attempts: #22222, #22226, #22260, #22440.

AI assistance: Codex helped diagnose, implement, and test this change. I reviewed the changes and tested them locally before submission.