fix(quaint): recognize json placeholders as json values

[aqrln]

Fixes some JSON tests failing for MySQL with parameterization enabled.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing aqrln-ttuomuyllppy (dc36758) with main (a09903a)}

Summary

✅ 11 untouched benchmarks
⏩ 11 skipped benchmarks¹

Footnotes

1. 11 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Copilot]

Pull request overview

This PR updates JSON expression detection so that JSON placeholders using ValueType::Opaque(OpaqueType::Json) are treated as JSON values/expressions, fixing MySQL JSON tests when parameterization is enabled.

Changes:

• Extend Expression::is_json_expr() to recognize parameterized values and rows whose opaque type is OpaqueType::Json as JSON expressions under the MySQL feature.
• Extend Expression::is_json_value() similarly, so JSON placeholders and rows backed by opaque JSON types are treated as JSON values.

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[github-actions]

Wasm Query Compiler File Size

Engine	This PR	Base branch	Diff
Postgres	3.485MiB	3.485MiB	308.000B
Postgres (gzip)	1.141MiB	1.141MiB	12.000B
Postgres (size-optimized)	1.758MiB	1.757MiB	66.000B
Postgres (size-optimized, gzip)	691.555KiB	691.600KiB	-46.000B
Mysql	3.437MiB	3.438MiB	-459.000B
Mysql (gzip)	1.122MiB	1.122MiB	-180.000B
Mysql (size-optimized)	1.727MiB	1.727MiB	147.000B
Mysql (size-optimized, gzip)	680.747KiB	680.622KiB	128.000B
Sqlite	3.356MiB	3.356MiB	0.000B
Sqlite (gzip)	1.095MiB	1.095MiB	0.000B
Sqlite (size-optimized)	1.682MiB	1.682MiB	0.000B
Sqlite (size-optimized, gzip)	664.495KiB	664.415KiB	82.000B
SQL Server	3.579MiB	3.579MiB	0.000B
SQL Server (gzip)	1.159MiB	1.159MiB	5.000B
SQL Server (size-optimized)	1.758MiB	1.758MiB	0.000B
SQL Server (size-optimized, gzip)	694.895KiB	694.898KiB	-3.000B
CockroachDB	3.535MiB	3.535MiB	308.000B
CockroachDB (gzip)	1.160MiB	1.160MiB	31.000B
CockroachDB (size-optimized)	1.781MiB	1.781MiB	66.000B
CockroachDB (size-optimized, gzip)	701.603KiB	701.518KiB	87.000B