fix: return schema of ExtensionPlan instead of its children's

[waynexia]

Which issue does this PR close?

Related to #5513, but not close it as there is another unfinished discussion.

Rationale for this change

#5236 returns chindren's schema rather than ExtensionPlan's in all_schema, which will break other places that make use of ExtensionPlan.

What changes are included in this PR?

Correct all_schema's behavior

Are these changes tested?

Are there any user-facing changes?

[jackwener]

Make great sense to me, thanks @waynexia .

Can you add a test to cover this case? thanks again.

[waynexia]

Make great sense to me, thanks @waynexia .

Can you add a test to cover this case? thanks again.

Appreciate your suggestion! I add a UT for this change, but only covers the ExtensionPlan branch 🥲

[jackwener]

Make great sense to me, thanks @waynexia .
Can you add a test to cover this case? thanks again.

Appreciate your suggestion! I add a UT for this change, but only covers the ExtensionPlan branch 🥲

I think we can add an extra integration-test to cover this case, look like unit test isn't enough.

[alamb]

I think this PR is an improvement and has test coverage. I agree with @jackwener we can probably improve the coverage but I also think we can do so in a follow on PR

[alamb]

Thanks @waynexia

[ursabot]

Benchmark runs are scheduled for baseline = 55c7c4d and contender = 6a8c4a6. 6a8c4a6 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java