refactor: move data file methods behind SupportsDataFiles trait

[william-ch-databricks]

Stacked PR

Use this link to review incremental changes.

• stack/alter-table-1-refactor-state [Files changed] [MERGED]
  • stack/alter-table-2-supports-data-files [Files changed]
    • stack/alter-table-3-framework-add-column [Files changed]
      • stack/alter-table-4-set-nullable [Files changed]
        • stack/alter-table-5-column-mapping-add [Files changed]
          • stack/alter-table-6-drop-column [Files changed]
            • stack/alter-table-7-rename-column [Files changed]
              • stack/alter-table-8-operation-string [Files changed]
                • stack/alter-table-9-is-blind-append [Files changed]

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What changes are proposed in this pull request?

Introduce a SupportsDataFiles marker trait to gate data-file methods at compile time. This prevents future metadata-only transaction types (like AlterTable) from accessing methods that add, remove, or configure data files.

Gated behind SupportsDataFiles (only ExistingTable and CreateTable):

Method	Reason
stats_schema()	Returns table-specific stats schema for writing files
stats_columns()	Returns column names needing stats collection
generate_logical_to_physical()	Builds partition column transform for file writing
get_write_context()	Returns full write context (target dir, schemas, column mapping)
add_files()	Queues file additions

Kept on impl<S> Transaction<S> (all transaction types):

Method	Reason
commit()	Shared commit path; handles empty data gracefully
with_data_change() / set_data_change()	ALTER TABLE sets data_change: false
with_engine_info()	Applicable to all commits
with_commit_info()	Applicable to all commits
with_transaction_id()	Applicable to all commits
with_domain_metadata()	Schema changes may need domain metadata
add_files_schema()	Read-only accessor; used by generate_adds() which handles empty state internally. Kept ungated so it remains available if it becomes dynamic in the future.
validate_add_files_stats()	Internal; handles empty data with early return
generate_adds()	Internal; returns early when no files queued
generate_remove_actions()	Internal; returns early when no files queued
generate_dv_update_actions()	Internal; returns early when no DV updates
generate_adds_for_dv_update()	Internal; only called from generate_dv_update_actions
build_crc_delta()	Internal; shared commit path
into_committed() / into_conflicted() / into_retryable()	Internal; shared commit path

How was this change tested?

All existing tests pass unchanged. The trait adds compile-time safety with no runtime behavior change. A compile_fail doctest will be added in the follow-up PR that introduces AlterTable to verify that data file methods are rejected at compile time.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (453d8c7) to head (a71c8b8).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

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  Coverage   88.40%   88.40%           
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  Files         175      175           
  Lines       57753    57753           
  Branches    57753    57753           
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  Hits        51059    51059           
  Misses       4750     4750           
  Partials     1944     1944           

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