feat(optimizer): improve index selection for primary lookup

[chenzl25]

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What's changed and what's your intention?

• Avoid index selection if the current plan is a primary lookup.
• Sysbench shows that this rule causes 3% CPU for the oltp_point_select workload.
• Related Sysbench performance bottleneck analysis -- point select #13459

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a3ea7c0) 68.18% compared to head (6b08e90) 68.19%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
...rontend/src/optimizer/rule/index_selection_rule.rs	82.14%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13658   +/-   ##
=======================================
  Coverage   68.18%   68.19%           
=======================================
  Files        1521     1521           
  Lines      261709   261724   +15     
=======================================
+ Hits       178459   178470   +11     
- Misses      83250    83254    +4     

Flag	Coverage Δ
rust	68.19% <82.14%> (+<0.01%)	⬆️

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