Add File Content Listener

[SachinAkash01]

Purpose

$subject

The current ftp:Listener reports file add/delete events via onFileChange, exposing metadata only. To process content, developers must invoke a client (Caller->get) inside the handler.
This PR adds content-first callbacks to the existing listener, so services can receive file content directly (stream, bytes, text, JSON, XML, CSV) as soon as a file is detected.

Fixes: ballerina-platform/ballerina-library#1490

Examples

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[codecov]

Codecov Report

❌ Patch coverage is 11.20815% with 610 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.13%. Comparing base (c0f8de8) to head (102d800).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
...stdlib/ftp/plugin/FtpContentFunctionValidator.java	0.00%	132 Missing ⚠️
...a/stdlib/ftp/server/FtpContentCallbackHandler.java	0.00%	131 Missing ⚠️
...ballerina/stdlib/ftp/util/FtpContentConverter.java	0.00%	91 Missing ⚠️
...ina/stdlib/ftp/plugin/FtpFileDeletedValidator.java	0.00%	85 Missing ⚠️
...va/io/ballerina/stdlib/ftp/server/FtpListener.java	18.84%	52 Missing and 4 partials ⚠️
...llerina/stdlib/ftp/server/ContentMethodRouter.java	14.75%	50 Missing and 2 partials ⚠️
...llerina/stdlib/ftp/plugin/FtpServiceValidator.java	27.02%	15 Missing and 12 partials ⚠️
...lerina/stdlib/ftp/util/FtpFileExtensionMapper.java	0.00%	15 Missing ⚠️
...ain/java/io/ballerina/stdlib/ftp/util/FtpUtil.java	52.17%	6 Missing and 5 partials ⚠️
...ballerina/stdlib/ftp/server/FtpListenerHelper.java	47.36%	6 Missing and 4 partials ⚠️

❌ Your project status has failed because the head coverage (62.13%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #1499       +/-   ##
=============================================
- Coverage     81.78%   62.13%   -19.65%     
- Complexity      272      392      +120     
=============================================
  Files            30       45       +15     
  Lines          1510     2525     +1015     
  Branches        194      416      +222     
=============================================
+ Hits           1235     1569      +334     
- Misses          197      812      +615     
- Partials         78      144       +66     

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[sonarqubecloud]

Quality Gate failed

Failed conditions
8.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[niveathika]

@SachinAkash01 I am merging this as it is urgent for the release, Lets track the disabled tests in different issue.

[niveathika]

The Module md file needs to be updated with latest docs

[niveathika]

For stream use cases, we shouldn't read the entire content to memory.