Fixes: #9988
Issue link: https://github.com/spring-projects/spring-integration/issues/9988
This change allows dynamic determination of `FileExistsMode` using SpEL expressions,
making the component more flexible when handling file existence conflicts.
* Add `fileExistsModeExpression` field and setter methods
* Use `resolveFileExistsMode()` in put and get operations
* Add changes to the docs
* Ignore `temporaryFileName` when `FileExistsMode.APPEND`
Improve runtime behavior by ignoring temporary filename settings when file exists mode is `APPEND`.
Now, in `FileExistsMode.APPEND` mode, content is always appended directly to the original file regardless of `useTemporaryFileName` setting.
In `RemoteFileTemplate`:
- Remove exception validation when `APPEND` mode is used with temporary filenames
- Modify logic to skip applying `temporaryFileSuffix` in `APPEND` mode
In `AbstractRemoteFileOutboundGateway`:
- Remove logic that disabled temporary filenames when setting `APPEND` mode
* Apply review feedback on `FileExistsMode` expression
- Optimize `EvaluationContext` usage by creating it once in `doInit()`
- Enhance expression evaluation to support String representation of `FileExistsMode`
- Optimize temporary filename handling logic in `RemoteFileTemplate`
- Add warning message for incompatible `APPEND` mode with temporary filenames
- Rename method to `setFileExistsModeExpressionString` for consistency
- Update Java DSL support in `RemoteFileOutboundGatewaySpec`
- Update reference documentation and release notes
* Apply additional review feedback on `FileExistsMode` expression
- Add `Function` variant to `RemoteFileOutboundGatewaySpec`
- Update documentations to use one-sentence-per-line style
- Improve code flow in `resolveFileExistsMode()` method
* Fix additional review feedback on `FileExistsMode` expression
- Use `Object` instead of `String` in `fileExistsModeFunction`
- Fix return method call in `fileExistsModeFunction` (`remoteDirectoryExpression` -> `fileExistsModeExpression`)
- Fix `standardEvaluationContext` initialization in `doInit()`
- Ensure proper `EvaluationContext` usage in other methods
Signed-off-by: Jooyoung Pyoung <pyoungjy@gmail.com>
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