CAMEL-18685: camel-file - add charsetUnmappable option to handle UnmappableCharacterException

[rpamu-jdev]

Summary

• Adds a new charsetUnmappable endpoint option with values REPORT (default), IGNORE, and REPLACE
• REPORT preserves existing behavior (throws UnmappableCharacterException)
• IGNORE silently drops unmappable/malformed characters
• REPLACE substitutes them with the charset replacement character

Changes

• IOHelper - new toCodingErrorAction() helper and overloads for toReader/toInputStream/EncodingInputStream accepting CodingErrorAction
• GenericFileEndpoint - new @UriParam option charsetUnmappable
• GenericFile - carries charsetUnmappable alongside charset
• FileConsumer - propagates setting when constructing GenericFile
• GenericFileConverter - applies CodingErrorAction when reading
• FileOperations - applies CodingErrorAction when writing

Usage

```java
from("file:inbox?charset=ISO-8859-1&charsetUnmappable=IGNORE")
from("file:inbox?charset=UTF-8").to("file:outbox?charset=ISO-8859-1&charsetUnmappable=REPLACE");
```

Fixes: https://issues.apache.org/jira/browse/CAMEL-18685

[github-actions]

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