feat(data-branch): materialize diff output as table

[gouhongshen]

Summary

Implements DATA BRANCH DIFF ... [COLUMNS (...)] OUTPUT AS <table>.

• Materializes an ordinary persistent physical table, not a branch node or session temporary table.
• Preserves requested visible-column order and types; adds source/flag metadata columns with collision-safe names.
• Requires read access to both diff inputs and CREATE TABLE (or database-wide equivalent) on the destination database.
• Rejects destination snapshots and leaves existing destination tables untouched.
• Cleans up a partially materialized destination on failure with a bounded cleanup timeout.

User impact

Diff output can now be retained and queried across sessions. The result table has no mo_branch_metadata row and is never treated as a data branch.

Validation

• mo-cgo-test -race -count=1 -timeout=240s ./pkg/frontend -run 'Test(DataBranchOutput|ValidateDataBranchDiffOutputAs)'
• go test ./pkg/sql/parsers/dialect/mysql -run 'TestDataBranchDiff(Columns|OutputModes)' -count=1
• make build
• BVT: diff_17 73/73 and branch privilege 257/257.
• Full git4data/branch/diff BVT: 1478/1478 passes with result-set metadata comparison disabled. Strict comparison reports 57 pre-existing metadata-only expectation mismatches in diff_8, diff_9, diff_12, diff_13, diff_14, and diff_16; every data row matches and the ordinary DIFF output path is unchanged. Those unrelated expected-result updates are intentionally excluded from this PR.