perf: preallocate sortStringExprs chunk slice size

[jbedard]

Buildtools PR checklist

• The code in this PR is covered by unit/integration tests.
• I have tested these changes and provide testing instructions below.
• I have either responded to, or resolved all Gemini comments on the PR.
• I have read Google Eng Practices on Small Changes, this PR either follows these guidelines or the description provides reasoning for why they can not be followed.

Description

The list size is guaranteed and can be pre-\allocatted.

[gemini-code-assist]

Code Review

This pull request optimizes slice allocation in build/rewrite.go by preallocating capacity for the chunk slice. The reviewer suggested a further optimization to initialize the slice with the exact length and assign elements directly by index to avoid the overhead of append.

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