ddl: split sessionctx.Context to sub contexts in CopContextBase

[lcwangchao]

What problem does this PR solve?

Issue Number: ref #53388

Problem Summary:

see description

What changed and how does it work?

This PR splits the sessionctx.Context in CopContextBase into several sub-contexts to make it easier to remove mock.Context in the further PRs:

• introduce NewReorgCopContext to create a copr.CopContext for reorg scene.
• NewReorgCopContext still uses newSessCtx to create a mock.Context but then several sub-contexts are fetched from it as the input argument of NewCopContext.
• The current PR does equivalent substitution so there will no logic change.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[lcwangchao]

Construct copCtx by ddl.NewReorgCopContext instead of copr.NewCopContextSingleIndex to make its inner state more like the one in production env.

[codecov]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 74.5500%. Comparing base (e560d03) to head (569b246).
Report is 6 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #53642        +/-   ##
================================================
+ Coverage   72.5723%   74.5500%   +1.9776%     
================================================
  Files          1506       1506                
  Lines        430741     431163       +422     
================================================
+ Hits         312599     321432      +8833     
+ Misses        98860      89787      -9073     
- Partials      19282      19944       +662     

Flag	Coverage Δ
integration	49.1588% <92.5000%> (?)
unit	71.4347% <87.5000%> (-0.1311%)	⬇️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	50.4810% <ø> (+8.6690%)	⬆️

[YangKeao]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tangenta, YangKeao

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• OWNERS [YangKeao,tangenta]
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-29 08:09:26.723813639 +0000 UTC m=+2850320.480949212: ☑️ agreed by tangenta.
• 2024-05-29 09:50:12.434118499 +0000 UTC m=+2856366.191254071: ☑️ agreed by YangKeao.