vreplication: preserve MySQL JSON types from binlog

[herbenderbler]

Description

VReplication was losing MySQL JSON internal types (DATE, TIME, DATETIME, etc.) on the binlog path: binary JSON was marshaled to plain text JSON before gRPC, and the replicator only re-encoded standard JSON literals on apply.
This PR keeps typed JSON through the pipeline by using MarshalSQLTo in the binlog reader and passing preserialized JSON SQL through replicator_plan instead of round-tripping via MarshalSQLValue.

Fixes #19880

Related Issue(s)

Fixes #19880

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

User-visible correctness fix for VReplication JSON columns with typed values (e.g. JSON_TYPE returns DATE instead of STRING after replicate). No schema or flag changes.

[vitess-bot]

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[Copilot]

Pull request overview

This PR fixes VReplication's loss of MySQL JSON internal type tags (DATE, TIME, DATETIME, DECIMAL, BIT, BLOB) by preserving the typed SQL form of JSON values end-to-end. On the binlog read path, CellValue now uses MarshalSQLTo (which emits JSON_OBJECT(...)/JSON_ARRAY(...)/CAST(... as JSON) expressions) instead of MarshalTo (which produced plain JSON text). The replicator's apply path is taught to recognize and pass through these pre-serialized SQL expressions via new helpers JSONSQLValue and AppendJSONSQL, falling back to MarshalSQLValue/AppendMarshalSQL for legacy text JSON (e.g., from initial table copy).

Changes:

• Switch binlog cell decoding to emit typed SQL JSON expressions (MarshalSQLTo).
• Add IsPreserializedJSONSQL, JSONSQLValue, and AppendJSONSQL helpers that detect SQL-form JSON by top-level prefix (JSON_OBJECT(/JSON_ARRAY(/CAST() and bypass re-encoding.
• Route the four VReplication apply call sites through the new helpers and add regression tests at all layers.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/mysql/binlog/rbr.go	Use MarshalSQLTo so JSON cell values carry SQL with type info.
go/mysql/binlog/rbr_test.go	Update expectations to SQL form; add DATE case.
go/mysql/json/marshal.go	New prefix-based detection plus JSONSQLValue / AppendJSONSQL.
go/mysql/json/marshal_test.go	Unit tests for new helpers and detection heuristic.
go/vt/vttablet/tabletmanager/vreplication/replicator_plan.go	Use new helpers at all JSON SQL-binding sites.
go/vt/vttablet/tabletmanager/vreplication/replicator_plan_test.go	Regression tests proving typed SQL is preserved.

[arthurschreiber]

This feels very hacky to me. 🤔

[herbenderbler]

This feels very hacky to me. 🤔

The prefix check is a workaround for type erasure on the VReplication wire path. binlog CellValue already returns sqltypes.Expression for typed JSON, but RowToProto3 only ships bytes and the target reconstructs the cell as Type_JSON. That being said, I can’t tell SQL expressions from text JSON without either a heuristic or extra metadata (similar to json_partial_values for partial updates). The prefixes match exactly what MarshalSQLTo emits. text JSON is rejected via {/[/".

I'm happy to switch to an explicit RowChange bitmap in a follow-up (or something else) if you’d prefer that over sniffing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.05%. Comparing base (70c7a72) to head (19b2d18).
⚠️ Report is 261 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #20120       +/-   ##
===========================================
+ Coverage   69.67%   78.05%    +8.38%     
===========================================
  Files        1614       37     -1577     
  Lines      216793     7623   -209170     
===========================================
- Hits       151044     5950   -145094     
+ Misses      65749     1673    -64076     

Flag	Coverage Δ
partial	78.05% <100.00%> (?)

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