fix(batch): surface background stream failures on exit instead of returning partial results

[g-despot]

Why

Split out of #2056: a behaviour change for client.batch.stream() / collection.batch.stream() on every platform, so it gets its own review. (#2056 stays open for reference; siblings: #2140 token-refresh fixes, #2142 the trimmed WASM transport.)

Today a background failure in the stream (the recv/loop task or thread dying, or the server ending the stream early) is only noticed if the user calls add_object() / flush() afterwards. If it happens after the last add, leaving the with block returns normally with partial results — silent data loss. The sync __exit__ never raised at all.

What changes

• _wait() (both sync and async) raises the recorded background exception, or a WeaviateBatchStreamError saying how many objects/references were left unsent when the background tasks/threads are gone. Partial results are still copied first, so batch.results / batch.failed_objects can be inspected after catching.
• Leaving with / async with batch.stream() raises that failure on a clean block. If the block itself raised, that exception wins and the background failure is only logged.
• Async flush() checks the background tasks each tick (the sync one already did), so a task that died without recording an exception raises instead of spinning forever. "Died unexpectedly" is a WeaviateBatchStreamError instead of a bare Exception.
• Sync _start() raises the stored background error as soon as a thread has died, instead of polling for 60 s and then blaming the network — defect 4 of Batch stream recovery closes the shared connection and can leave the client permanently closed #2139 (the __reconnect shared-connection defects 1–3 there are not addressed here).
• _BatchStreamShutdownError (gRPC ABORTED) is now a WeaviateBatchStreamError subclass, since it can reach users on exit.

What users will notice

with client.batch.stream() as b:
    ...
print(client.batch.failed_objects)

may now raise WeaviateBatchStreamError at the end of the block where it previously returned silently with unsent data. That is the point, but it is a behaviour change.

Tests

test/test_batch_stream_async.py (4) / test/test_batch_stream_sync.py (5), unit tests over the private internals, no cluster: _wait raises and keeps partial results, unsent-data message, flush with a dead task, _start with a stored error (fails on main after the 60 s timeout), and the context-manager rules (clean block raises; the user's exception wins). test/ + mock_tests/test_batch.py pass locally; ruff / flake8 / pyright clean.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.29932% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.66%. Comparing base (95b5d76) to head (f2e3e11).
⚠️ Report is 79 commits behind head on main.

Files with missing lines	Patch %	Lines
weaviate/collections/batch/async_.py	90.47%	2 Missing ⚠️
test/test_batch_stream_async.py	99.31%	1 Missing ⚠️
weaviate/collections/batch/batch_wrapper.py	94.44%	1 Missing ⚠️
weaviate/exceptions.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2141      +/-   ##
==========================================
+ Coverage   86.64%   88.66%   +2.01%     
==========================================
  Files         300      306       +6     
  Lines       23172    23757     +585     
==========================================
+ Hits        20077    21063     +986     
+ Misses       3095     2694     -401     

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