fix(core): migrate FileSerde to binary ION API

[nkwiatkowski]

Summary

• Migrate FileTransform and its tests from deprecated Reader/Writer-based FileSerde methods to the new InputStream/OutputStream API
• This enables binary ION serialization for Kestra 2.0 (~20-40% storage reduction)
• Bump kestraVersion from 1.3.16 to 1.3.19 (includes the InputStream/OutputStream compatibility layer)

Changes

• FileTransform.java: BufferedWriter/FileWriter → BufferedOutputStream/FileOutputStream, BufferedReader/InputStreamReader → BufferedInputStream, Writer → OutputStream parameter
• FileTransformTest.java: BufferedReader/InputStreamReader → BufferedInputStream, FileSerde.reader() → FileSerde.read(), proper try-with-resources on input streams
• gradle.properties: kestraVersion 1.3.16 → 1.3.19

Test plan

• ./gradlew :plugin-script:compileJava :plugin-script:compileTestJava passes
• CI tests pass
• QA validation before rolling out to other plugins

Refs: kestra-io/kestra#3155

[github-actions]

📦 Artifacts

Name	Size	Updated	Expiration
	56.62 MB	May 22, 26, 1:17:00 PM UTC	May 29, 26, 1:16:57 PM UTC

🧪 Java Unit Tests

	Tests	Passed ✅	Skipped	Failed	Time ⏱
Java Tests Report	218 ran	218 ✅	0 ⚠️	0 ❌	19m 58s 625ms

🔁 Unreleased Commits

✅ No unreleased commits found.

[github-actions]

Tests report quick summary:

success ✅ > tests: 218, success: 218, skipped: 0, failed: 0

unfold for details

Project	Status	Success	Skipped	Failed
plugin-script-bun	success ✅	3	0	0
plugin-script-deno	success ✅	3	0	0
plugin-script-go	success ✅	19	0	0
plugin-script-groovy	success ✅	8	0	0
plugin-script-jbang	success ✅	2	0	0
plugin-script-julia	success ✅	2	0	0
plugin-script-jython	success ✅	5	0	0
plugin-script-lua	success ✅	3	0	0
plugin-script-nashorn	success ✅	6	0	0
plugin-script-node	success ✅	13	0	0
plugin-script-perl	success ✅	3	0	0
plugin-script-php	success ✅	3	0	0
plugin-script-python	success ✅	61	0	0
plugin-script-powershell	success ✅	4	0	0
plugin-script-r	success ✅	2	0	0
plugin-script-ruby	success ✅	34	0	0
plugin-script-shell	success ✅	47	0	0