[source-mysql] Concurrent initial-load partitioning generates non-latin1 boundary strings for varchar PK on latin1 table → error 3988 "Conversion from collation utf8mb4_0900_ai_ci into latin1_swedish_ci impossible for parameter"

### Connector Name

source-mysql

### Connector Version

3.53.1

### What step the error happened?

During the sync

## Environment

- Airbyte OSS 1.6.0 (Helm deployment on EKS)
- source-mysql 3.53.1
- destination-snowflake 4.0.4
- MySQL 8.0.42 (Aurora RDS)
- Sync mode: Incremental | Append + Deduped, CDC (read changes using binary log)

## Relevant information

The initial snapshot of one specific table fails on every attempt (4/4) with:

```
java.sql.SQLException: Conversion from collation utf8mb4_0900_ai_ci into latin1_swedish_ci impossible for parameter
```

The failing table is large (~13.6 GB estimated) and has a **`varchar(128)` primary key on a table with `DEFAULT CHARSET=latin1`** (PK collation `latin1_swedish_ci`):

```sql
CREATE TABLE `person` (
`personID` varchar(128) NOT NULL,
-- ... ~45 more columns ...
PRIMARY KEY (`personID`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
```

### Root cause analysis

Because the table exceeds the 3000 MiB target partition size, `MySqlJdbcConcurrentPartitionsCreator` splits the snapshot into 6 partitions:

```
i.a.i.s.m.MySqlJdbcConcurrentPartitionsCreator(run):521 Table memory size estimated at 13632 MiB.
i.a.i.s.m.MySqlJdbcConcurrentPartitionsCreator(run):522 Target partition size is 3000 MiB.
i.a.i.s.m.MySqlJdbcConcurrentPartitionsCreator(run):551 Table will be read by 6 concurrent partition reader(s).
```

Since the PK is a string, `MySqlSourceJdbcPartitionFactory.internalCalculateBoundaries` synthesizes midpoint boundary strings by interpolating between the sampled min/max keys. The real bounds are plain ASCII, but the four computed midpoints contain arbitrary Unicode code points, including supplementary-plane characters:

```
i.a.i.s.m.MySqlSourceJdbcPartitionFactory(internalCalculateBoundaries):702 boundaries:
[person7cry,
person?a𶛓򣎦]𶛐񬵃𶚫񬴼򣎠򣎨𶛓𶚯񬴼𶚽򣎓𶛐󙧽򣎈c𶚰򣎋𶚥𶚭򣎟5E_...,
personG_񬴵𶛓d񬴺󙨐񬴓󙨊𶛎𶛚...,
personO]򣎖󙨀k򣎥𶛝...,
personW[󙧸񬴭r󙨏򣎪...,
person_ZZZ]
```

These synthetic strings are then bound as prepared-statement parameters:

```
i.a.c.r.JdbcSelectQuerier$Result():79 Querying SELECT ... FROM `folkdata`.`person` WHERE (`personID` >= ?) AND (`personID` <= ?)
i.a.c.r.JdbcSelectQuerier$Result(initQueryExecution):103 Setting parameter #2 to Binding(value="person?a𶛓򣎦]𶛐...", type=StringFieldType)
```

The JDBC connection charset is utf8mb4, so MySQL 8 attempts to coerce the `utf8mb4_0900_ai_ci` parameter into the column's `latin1_swedish_ci` collation (to use the PK index) and fails with server error 3988, because the interpolated code points are not representable in latin1.

### Stack trace

```
java.sql.SQLException: Conversion from collation utf8mb4_0900_ai_ci into latin1_swedish_ci impossible for parameter
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:121)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:114)
at com.mysql.cj.jdbc.ServerPreparedStatement.serverExecute(ServerPreparedStatement.java:612)
at com.mysql.cj.jdbc.ServerPreparedStatement.executeInternal(ServerPreparedStatement.java:320)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeQuery(ClientPreparedStatement.java:1056)
at io.airbyte.cdk.read.JdbcSelectQuerier$Result.initQueryExecution(SelectQuerier.kt:107)
at io.airbyte.cdk.read.JdbcSelectQuerier$Result.(SelectQuerier.kt:81)
at io.airbyte.cdk.read.JdbcSelectQuerier.executeQuery(SelectQuerier.kt:58)
at io.airbyte.integrations.source.mysql.MySqlSourceSelectQuerier.executeQuery(MySqlSourceSelectQuerier.kt:31)
at io.airbyte.cdk.read.JdbcNonResumablePartitionReader.run(JdbcPartitionReader.kt:131)
at io.airbyte.cdk.read.FeedReader$readPartitionWithResources$4.invokeSuspend(FeedReader.kt:283)
...
```

`failureOrigin: source` on all 4 attempts; the retries can never succeed because the boundary interpolation deterministically produces non-latin1 code points.

### Conditions to reproduce

1. A table large enough to trigger the concurrent partition split (> ~3000 MiB estimated), AND
2. a string (varchar) primary key on a column whose charset is latin1 (or any charset smaller than utf8mb4), AND
3. PK values that are not GUID-shaped (so the GUID-specific handling from #69104 / #76050 does not apply — our keys look like `person`).

Setting connector concurrency to 1 does **not** avoid the bug — the log shows `Effective concurrency: 1` yet the table is still split into 6 partitions, and each partition query still uses the synthetic boundaries.

### Expected behavior

Partition boundary strings should be usable in comparisons against the PK column. Possible fixes:

- Clamp interpolated code points to the column's character set (for latin1, code points ≤ U+00FF), or
- use actual sampled key values as boundaries instead of synthetic midpoints (as source-mysql 3.11.x did), or
- wrap the parameter in `CONVERT(? USING )` / add an explicit `COLLATE` so the server can compare without coercing the parameter.

### Workaround

Downgrading the connection's source-mysql to 3.11.21 avoids the issue (its chunking uses real key values). This matches the workaround reported for the related partition-boundary regressions in #64909 / #64139.

### Related issues

- #64909 ([source-mysql] Regression in 3.50.x: ArithmeticException in partition boundary calculation)
- #64139 ([source-mysql] java.lang.ArithmeticException: / by zero)
- #69104 (Better partitioning for tables with GUID string primary key — does not cover non-GUID string PKs)

### Relevant log output

```shell
java.sql.SQLException: Conversion from collation utf8mb4_0900_ai_ci into latin1_swedish_ci impossible for parameter
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:121)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:114)
at com.mysql.cj.jdbc.ServerPreparedStatement.serverExecute(ServerPreparedStatement.java:612)
at com.mysql.cj.jdbc.ServerPreparedStatement.executeInternal(ServerPreparedStatement.java:320)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeQuery(ClientPreparedStatement.java:1056)
at io.airbyte.cdk.read.JdbcSelectQuerier$Result.initQueryExecution(SelectQuerier.kt:107)
at io.airbyte.cdk.read.JdbcSelectQuerier$Result.(SelectQuerier.kt:81)
at io.airbyte.cdk.read.JdbcSelectQuerier.executeQuery(SelectQuerier.kt:58)
at io.airbyte.integrations.source.mysql.MySqlSourceSelectQuerier.executeQuery(MySqlSourceSelectQuerier.kt:31)
at io.airbyte.cdk.read.JdbcNonResumablePartitionReader.run(JdbcPartitionReader.kt:131)
at io.airbyte.cdk.read.FeedReader$readPartitionWithResources$4.invokeSuspend(FeedReader.kt:283)
...
```

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**Internal Tracking:** airbytehq/oncall#13104