fix(dataset): preserve push_data compatibility with the released Apify SDK

[vdusek]

The scheduled E2E tests fail on every matrix job (all 90) with:

AttributeError: 'ApifyDatasetClient' object has no attribute '_is_sequence_of_items'

DatasetClient._is_sequence_of_items was removed in #2145. The released Apify SDK (up to 4.0.1) still calls it from ApifyDatasetClient.push_data, inheriting it from this base class, so a master wheel paired with the published SDK raises on every push_data. The templates E2E test builds exactly that pairing. Published crawlee 1.9.1 predates the removal, so this is not only a CI problem: the next crawlee release would carry the same break to every Actor resolving apify<=4.0.1.

This restores the helper on DatasetClient, annotated -> bool rather than TypeIs[...]. The narrowing is what ty 0.0.69 objected to in the first place, and no push_data implementation here uses the helper anymore, they keep the inlined isinstance. Drop it in the next major version, once those SDK versions are out of the supported range.

Verified by installing the built wheel alongside apify==4.0.1 and confirming the SDK call site resolves; swapping in the unpatched base class reproduces the failure above.

Failing run: https://github.com/apify/crawlee-python/actions/runs/31554176158

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.64%. Comparing base (7f75295) to head (dbb267d).

Files with missing lines	Patch %	Lines
...c/crawlee/storage_clients/_base/_dataset_client.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2152      +/-   ##
==========================================
+ Coverage   93.62%   93.64%   +0.02%     
==========================================
  Files         181      181              
  Lines       12649    12653       +4     
==========================================
+ Hits        11842    11849       +7     
+ Misses        807      804       -3     

Flag	Coverage Δ
unit	93.64% <75.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Pijukatel]

Stagehand templates are still failing (Maybe another issue)
https://github.com/apify/crawlee-python/actions/runs/31583394515/job/94071500831

[vdusek]

Stagehand:

ImportError: cannot import name 'AsyncStagehand' from 'stagehand'
(/usr/local/lib/python3.13/site-packages/stagehand/init.py)

So it's not related to this one, I'll resolve it as separate PR, and same for the alerting.