fix(watermark): fix merge watermark on scale

[BugenZhao]

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What's changed and what's your intention?

Fix unwrap panic when scaling the BufferedWatermark and add a simple test for that.

• Close bug: watermark buffer panics when scaling  #8203

Checklist For Contributors

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• I have demonstrated that backward compatibility is not broken by breaking changes and created issues to track deprecated features to be removed in the future. (Please refer to the issue)
• All checks passed in ./risedev check (or alias, ./risedev c)

Checklist For Reviewers

• I have requested macro/micro-benchmarks as this PR can affect performance substantially, and the results are shown.

Documentation

• My PR DOES NOT contain user-facing changes.

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

[wangrunji0408]

The testing part LGTM.

[yuhao-su]

The watermark part LGTM

[codecov]

Codecov Report

Merging #8223 (e2f448b) into main (a28f711) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##             main    #8223   +/-   ##
=======================================
  Coverage   71.57%   71.58%           
=======================================
  Files        1131     1131           
  Lines      182817   182822    +5     
=======================================
+ Hits       130859   130876   +17     
+ Misses      51958    51946   -12     

Flag	Coverage Δ
rust	71.58% <83.33%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/stream/src/executor/watermark/mod.rs	64.17% <78.57%> (+2.27%)	⬆️
src/stream/src/executor/hash_join.rs	97.31% <100.00%> (+<0.01%)	⬆️
src/storage/src/hummock/compactor/mod.rs	80.38% <0.00%> (-0.20%)	⬇️
src/common/src/cache.rs	97.26% <0.00%> (+0.10%)	⬆️
...rc/storage/hummock_sdk/src/filter_key_extractor.rs	93.89% <0.00%> (+0.25%)	⬆️
src/batch/src/executor/group_top_n.rs	74.70% <0.00%> (+6.47%)	⬆️

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