GH-9988: Add FileExistsMode expression support

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>

src/reference/antora/modules/ROOT/pages/ftp/rft.adoc

@@ -37,3 +37,21 @@ This is useful when you need to perform several high-level operations of the `Re
 For example, `AbstractRemoteFileOutboundGateway` uses it with the `mput` command implementation, where we perform a `put` operation for each file in the provided directory and recursively for its sub-directories.
 See the https://docs.spring.io/spring-integration/api/org/springframework/integration/file/remote/RemoteFileOperations.html#invoke[Javadoc] for more information.
 
+Starting with version 6.5, the `AbstractRemoteFileOutboundGateway` supports dynamic resolution of `FileExistsMode` at runtime via SpEL expressions.
+This allows you to determine the action to take when files already exist based on message content or other conditions.
+
+To use this feature, configure the `fileExistsModeExpression` property on the gateway.
+The expression can evaluate to:
+
+* A `FileExistsMode` enum value (e.g., `FileExistsMode.REPLACE`)
+* A string representation of a `FileExistsMode` (case-insensitive, e.g., "REPLACE", "append")
+
+If the expression returns `null`, the default `fileExistsMode` configured on the gateway will be used.
+
+See the https://docs.spring.io/spring-integration/api/org/springframework/integration/file/remote/gateway/AbstractRemoteFileOutboundGateway.html#setFileExistsModeExpression(org.springframework.expression.Expression)[Javadoc] for more information.
+
+[IMPORTANT]
+====
+When using `FileExistsMode.APPEND`, temporary filename functionality is automatically disabled regardless of the `useTemporaryFileName` setting.
+This is because appending to a temporary file and then renaming it would not achieve the intended append behavior.
+====

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -79,6 +79,12 @@ The `AbstractRecentFileListFilter` strategy has been introduced to accept only t
 The respective implementations are provided: `RecentFileListFilter`, `FtpRecentFileListFilter`, `SftpRecentFileListFilter` and `SmbRecentFileListFilter`.
 See xref:file/reading.adoc[Reading Files] for more information.
 
+[[x6.5-file-exists-mode-expression]]
+== FileExistsMode Expression Support
+
+The remote file gateways (`AbstractRemoteFileOutboundGateway`) now support dynamic resolution of `FileExistsMode` at runtime via SpEL expressions.
+See xref:ftp/rft.adoc[Remote File Gateways] for more information.
+
 [[x6.5-hazelcast-changes]]
 == Hazelcast Module Deprecations