Fix Automation skill completion interactions

[Paul (pwang347)]

Follow-up to #333669.

Fix skill completion interactions in the Automations dialog so that:

• Tab and Enter accept a focused skill suggestion instead of being intercepted by dialog navigation or newline handling.
• Accepted skills use the same inline reference styling as regular chat inputs.
• Saved skill references are revalidated against the active Agent Host and highlighted again when an Automation is reopened.
• Edited or invalid skill references lose their decoration when their text no longer matches.
• Runtime skill completion metadata continues to be preserved end-to-end.

The shared chat reference decoration registration is extracted so regular chat and Automations use the same theme-aware styling. Automations still persists a plain prompt string rather than chat attachment state.

Validation

• npm run transpile-client
• Focused unit tests across Automations, chat variables, Agent Host metadata/completion mapping, and chat contribution behavior: 504 passing
• Latest Automation interaction and persistence suite: 35 passing

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: 1 · 1

New issues introduced by this change (2)

Severity	Finding
	src/​vs/​sessions/​contrib/​automations/​browser/​automationDialog.ts — Returning here still does not let Monaco handle Tab. This listener is installed during…
	src/​vs/​sessions/​contrib/​automations/​browser/​automationInputCompletions.ts — This equality check misses edits at either edge of the reference. The decoration uses…

What changed in this PR

Updates Automations skill completion interactions and shares chat reference styling.

Changes:

• Adjusts Tab handling when suggestions are focused.
• Adds and maintains inline skill-reference decorations.
• Adds focused completion and keyboard-navigation tests.

File	Description
chatInputReferenceDecorations.ts	Extracts shared decoration registration.
chatInputEditorContrib.ts	Uses the shared decoration helper.
automationInputCompletions.ts	Tracks accepted skill decorations.
automationDialog.ts	Exposes suggestion state to keyboard navigation.
automationDialogService.ts	Wires suggestion state into the focus trap.
automationInputCompletions.test.ts	Tests completion decoration creation.
automationDialog.test.ts	Tests Tab propagation with suggestions.

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