planner: Add a framework for disabling the plan cache during optimization.

[wddevries]

What problem does this PR solve?

Issue Number: ref #65895
Problem Summary:

There is no universal way to disable the plan cache for a query during optimization. The two existing methods have limitations:

1. SetSkipPlanCache disables the plan cache preemptively, even if the problematic plan is not ultimately selected.
2. isPhysicalPlanCacheable walks the final plan to check cacheability, but not all conditions can be discovered at this stage since context from earlier optimization phases may be lost.

What changed and how does it work?

This PR introduces a general framework for tracking and propagating non-cacheable reasons through the planner:

1. Plan interface changes (pkg/planner/core/base/plan_base.go): Added SetNoncacheableReason and GetNoncacheableReason methods to the Plan interface. Any component in the planner can now mark a plan as non-cacheable with a specific reason.
2. Base Plan implementation (pkg/planner/core/operator/baseimpl/plan.go): Added NotCachableReason field to the base Plan struct with getter/setter implementations. The setter only sets the reason if one isn't already set, preserving the first/root cause.
3. AccessPath support (pkg/planner/util/path.go): Added NoncacheableReason field to AccessPath as one source of non-cacheable reasons, allowing path-level decisions to propagate to plans.
4. Propagation during plan building: Updated plan building functions to propagate NoncacheableReason from AccessPath to resulting physical plans (index scans, point gets, index merge paths, etc.).
5. Plan cache check (pkg/planner/core/plan_cacheable_checker.go): Added check in isPhysicalPlanCacheable to return false with the specific reason if a plan has a non-cacheable reason set.
6. Refactoring (pkg/planner/core/indexmerge_unfinished_path.go): Changed unfinishedAccessPath to store the full AccessPath instead of just the index, enabling NoncacheableReason to flow through index merge path building.

Check List

Tests

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

• This is framework/infrastructure code that will be exercised by follow-up PRs that set actual non-cacheable reasons

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 (​https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/release-notes-style-guide.html​) to write a quality release note.

None

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[codecov]

Codecov Report

❌ Patch coverage is 85.41667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.7216%. Comparing base (245c8e1) to head (ac8a886).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65896        +/-   ##
================================================
- Coverage   77.7927%   77.7216%   -0.0712%     
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  Files          2001       1923        -78     
  Lines        545191     534877     -10314     
================================================
- Hits         424119     415715      -8404     
+ Misses       119410     119138       -272     
+ Partials       1662         24      -1638     

Flag	Coverage Δ
integration	41.4777% <74.4680%> (-6.6918%)	⬇️
unit	76.8050% <85.4166%> (+0.3755%)	⬆️

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Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8701% <ø> (-12.1038%)	⬇️

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[wddevries]

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[wddevries]

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[wddevries]

/test unit-test

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