chore: avoid reparsing partition expr in mito2 reader

[lyang24]

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What's changed and what's your intention?

in the read path there might be a opportunity to save a call on get region_partition_expr which link to parse_partition_expr with some json work.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[gemini-code-assist]

Code Review

This pull request refactors the ParquetReaderBuilder in src/mito2/src/sst/parquet/reader.rs by lifting the computation of region partition expressions out of the build_partition_filter method. The method signature is updated to accept these values as arguments, which are now passed from the caller where they were already being determined. I have no feedback to provide.