[BUG] A2A artifacts parts are reassembled to corrupted text with extra whitespaces

Description

When a remote A2A agent streams (or persists) its reply as more than one
text chunk, CrewAI's client-side reassembly corrupts the text: words get
split apart and extra spaces are inserted.
Per the A2A spec, artifact parts sent with append=True are meant to be
concatenated directly with no separator — that's the definition of
"append". CrewAI instead does:

# crewai/a2a/task_helpers.py, process_task_state()
response_text = " ".join(result_parts) if result_parts else ""

This function is shared by all three update mechanisms
(StreamingHandler, PollingHandler, PushNotificationHandler), so
switching updates= doesn't avoid it — and there's an identical
" ".join(result_parts) fallback inside StreamingHandler.execute()
itself (crewai/a2a/updates/streaming/handler.py), plus a third,
independent occurrence with the same pattern in
crewai/a2a/wrapper.py::_handle_max_turns_exceeded.
This affects any spec-compliant server that streams its reply
incrementally — e.g. any server relaying real token-by-token LLM output —
regardless of framework. It isn't specific to one server implementation.

Steps to Reproduce

"""Minimal, framework-agnostic reproduction of a CrewAI A2A bug.
Run:
    pip install crewai a2a-sdk
    python repro_a2a_streaming_join_bug.py
Expected: Hello, world!
Actual:   Hel lo,  wor ld !
"""
from __future__ import annotations
import asyncio
import uuid
from a2a.types import (
    AgentCapabilities,
    AgentCard,
    Artifact,
    Message,
    Part,
    Role,
    Task,
    TaskArtifactUpdateEvent,
    TaskState,
    TaskStatus,
    TaskStatusUpdateEvent,
    TextPart,
)
from crewai.a2a.updates.streaming.handler import StreamingHandler
TASK_ID = "task-1"
CONTEXT_ID = "ctx-1"
# Split "Hello, world!" into several small append=True chunks -- exactly
# what any server relaying incremental/token-level output would send.
CHUNKS = ["Hel", "lo, ", "wor", "ld", "!"]
EXPECTED = "".join(CHUNKS)
def _text_part(text: str) -> Part:
    return Part(root=TextPart(text=text))
async def _fake_send_message(_message: Message):
    """Stand-in for `a2a.client.Client.send_message`, yielding (Task, update)
    tuples the same way a real client does while consuming a server's SSE
    stream."""
    task = Task(id=TASK_ID, context_id=CONTEXT_ID, status=TaskStatus(state=TaskState.working))
    for i, chunk in enumerate(CHUNKS):
        artifact = Artifact(artifact_id="reply", parts=[_text_part(chunk)])
        update = TaskArtifactUpdateEvent(
            task_id=TASK_ID,
            context_id=CONTEXT_ID,
            artifact=artifact,
            append=i > 0,
            last_chunk=i == len(CHUNKS) - 1,
        )
        yield (task, update)
    final_status = TaskStatusUpdateEvent(
        task_id=TASK_ID,
        context_id=CONTEXT_ID,
        status=TaskStatus(state=TaskState.completed),
        final=True,
    )
    yield (task, final_status)
class FakeClient:
    def send_message(self, message: Message):
        return _fake_send_message(message)
async def main() -> None:
    agent_card = AgentCard(
        name="Repro Agent",
        description="Minimal agent card for reproduction purposes.",
        url="http://localhost:9999",
        version="1.0.0",
        capabilities=AgentCapabilities(streaming=True),
        default_input_modes=["text"],
        default_output_modes=["text"],
        skills=[],
    )
    message = Message(
        role=Role.user,
        parts=[_text_part("say hello")],
        message_id=str(uuid.uuid4()),
    )
    result = await StreamingHandler.execute(
        client=FakeClient(),
        message=message,
        new_messages=[],
        agent_card=agent_card,
        endpoint=agent_card.url,
    )
    actual = result["result"]
    print(f"Expected: {EXPECTED!r}")
    print(f"Actual:   {actual!r}")
    assert actual == EXPECTED, "BUG: chunks were joined with a space instead of concatenated"
if __name__ == "__main__":
    asyncio.run(main())

Expected behavior

Output:

Expected: 'Hello, world!'
Actual:   'Hel lo,  wor ld !'
Traceback (most recent call last):
  ...
AssertionError: BUG: chunks were joined with a space instead of concatenated

Screenshots/Code snippets

# crewai/a2a/task_helpers.py, process_task_state()
response_text = " ".join(result_parts) if result_parts else ""

Operating System

macOS Sonoma

Python Version

3.11

crewAI Version

1.15.21

crewAI Tools Version

n/a

Virtual Environment

Venv

Evidence

see above

Possible Solution

just reassemble with "".join instead of " ".join ?

Additional context

no

Run the supplied minimal reproduction first, then inspect process_task_state() in crewai/a2a/task_helpers.py and the fallback joins in crewai/a2a/updates/streaming/handler.py and crewai/a2a/wrapper.py. Confirm that all three update paths preserve the A2A append semantics, and rerun the reproduction to verify the result is exactly 'Hello, world!' without inserted spaces.