refactor(mito2): remove PrimaryKey variants

[evenyag]

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Refer to a related PR or issue link (optional)

#7732

What's changed and what's your intention?

Remove the old PrimaryKey read format variants and make the scan/read path use the flat format exclusively. This is a cleanup PR that drops ~1800 lines of dead code after the flat format became the only active path.

Summary of changes:

• Make the scan compat path flat-only, removing PrimaryKey variants from ReadFormat, BatchReader, and related enums
• Use FlatProjectionMapper directly instead of the ProjectionMapper wrapper that dispatched between flat and primary-key modes
• Drop dead primary-key projection helpers from projection.rs and prune.rs
• Remove dead compat batch enum (Batch vs FlatBatch distinction)
• Use ReadFormat::Flat directly in the SST parquet reader and file range code, simplifying ParquetReaderBuilder, FileRangeContextBuilder, and format helpers
• Fix resulting compiler warnings

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 mito2 engine to remove legacy primary key format reading paths and projection mappers, consolidating the codebase around the flat format. Key changes include the removal of the ReadFormat enum in favor of FlatReadFormat, the replacement of ProjectionMapper with FlatProjectionMapper, and the deletion of associated compatibility layers such as CompatReader and PrimaryKeyCompatBatch. Additionally, PruneReader and RowGroupReader have been removed or simplified, and several utility functions have been transitioned to top-level functions. I have no feedback to provide.