*: extend DXF collation handling to support partition table and partial index

[joechenrh]

What problem does this PR solve?

Issue Number: close #69563

Problem Summary:

In next-gen deployments, DXF workers may run in a keyspace whose new-collation setting differs from the user keyspace that submitted the task. Some remaining ADD INDEX and IMPORT INTO paths still built expressions, encoded keys, or converted string-like values using the worker/global collation mode. This can make background execution use collation semantics that do not match the submitted task.

What changed and how does it work?

This PR keeps the new-collation mode captured by the submitted task/table snapshot and uses it in the remaining collation-sensitive DXF paths:

• Introduce table.EncodingConfig as the explicit encoding snapshot carried by table/index/expression helpers.
• Keep the normal TableFromMeta(allocs, tblInfo) path unchanged, and let DXF paths pass tables.WithEncodingConfig(...) when they need a fixed task snapshot.
• Build partition expressions and list partition encoded keys with the table collation snapshot.
• Build partial-index backfill conditions with the collation mode carried by the coprocessor context.
• Make BuildSimpleExpr honor WithUseNewCollate when it builds schema from WithTableInfo.
• Add explicit ENUM/SET parsing and datum conversion helpers, and use the table collation snapshot in IMPORT/Lightning cast and generated-column evaluation paths.
• Add focused regression tests for mismatched global/table collation modes.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Validated locally:

make bazel_prepare
go test ./pkg/types -run 'Test(Enum|Set)$' -count=1
go test ./pkg/ddl/copr -run 'Test(NewCopContextSingleIndex|CopContextConditionUsesFixedCollation)' -count=1
./tools/check/failpoint-go-test.sh pkg/planner/core -run TestBuildSimpleExprWithTableInfoUsesFixedCollation -count=1
./tools/check/failpoint-go-test.sh pkg/executor/importer -run 'TestKVEncoderCast(Enum|EnumSet)' -count=1
./tools/check/failpoint-go-test.sh pkg/table/tables/test/partition -run TestPartitionTableUsesTableCollationSnapshot -count=1
go test -run '^$' ./pkg/ddl/copr ./pkg/lightning/backend/kv ./pkg/table -count=1
./tools/check/failpoint-go-test.sh pkg/table/tables -run '^$' -count=1
go test -run '^$' ./pkg/dxf/importinto -count=1
./tools/check/failpoint-go-test.sh pkg/table/tables -run TestTableFromMetaEncodingConfigOptionUsesFixedMode -count=1
./tools/check/failpoint-go-test.sh pkg/ddl -run '^$' -count=1
git diff --check
make lint

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix an issue that DXF ADD INDEX or IMPORT INTO might use the worker keyspace's new-collation setting instead of the submitted task's setting in some collation-sensitive paths.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
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