[Inference Endpoints] Fix cursor jumping when editing Inference Endpoint ID

[spong]

Summary

• Fixes a controlled-input bug in the Add/Edit inference endpoint flyout where editing the Inference endpoint ID mid-string inserted one character, then jumped the cursor to the end.
• Root cause: the input was bound to a parent useFormData snapshot (config.inferenceId) while updates went through setFieldValue, so UseField re-rendered one frame with a stale value.
• Bind the input to field.value / field.onChange (the form lib’s supported pattern) and add a regression test that asserts cursor position is preserved.

Before

Screen.Recording.2026-07-23.at.14.20.45.mov

After

Screen.Recording.2026-07-23.at.14.51.00.mov

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• Review the backport guidelines and apply applicable backport:* labels.

PR Developed with Cursor + Grok-4.5

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 84ef6b3
• Build duration: 72 mins

Failed CI Steps

• FTR Configs #58
• Scout Lane #10 - stateful-classic / default

Test Failures

• [job] [logs] Scout Lane #10 - stateful-classic / default / local-stateful-classic - Entity attachment cases – flyout add-to-case actions and Entities accordion - Entities accordion renders when entity attachments were added via API
• [job] [logs] Scout Lane #10 - stateful-classic / default / local-stateful-classic - Entity attachment cases – flyout add-to-case actions and Entities accordion - renders no Entities accordion when a case has no entity attachments
• [job] [logs] FTR Configs #58 / machine learning - short tests model management trained models add trained model flyout for ML power user should contain a Click to Download and a Manual Download tab

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
indexManagement	1.0MB	1.0MB	-87.0B
observabilityAIAssistantApp	680.5KB	680.4KB	-87.0B
searchAssistant	200.8KB	200.7KB	-87.0B
searchInferenceEndpoints	251.1KB	251.0KB	-87.0B
stackConnectors	801.6KB	801.5KB	-87.0B
total			-435.0B

cc @spong