chat: support inline attachment references

[justschen]

• Lets users mention context that is already attached so prompts can express where a file, folder, image, pasted text, or other attachment is relevant without duplicating or removing it.
• Keeps inline reference state lightweight so large attachment payloads do not cause typing lag, while preserving the original payload when the request is sent.
• Prioritizes attached context in # and @ suggestions so existing context is easy to discover and reference.
• Adds accessibility guidance and regression coverage for ordering, payload preservation, request ranges, and compact reference state.

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[Copilot]

Pull request overview

Adds inline references to existing chat attachments while preserving payloads and minimizing typing-state overhead.

Changes:

• Adds compact attachment-reference state and request-range restoration.
• Prioritizes attached context in # and @ completions.
• Adds accessibility guidance and regression tests.

Show a summary per file

File	Description
chatRequestParser.test.ts	Tests payload and range preservation.
chatVariableEntries.test.ts	Tests immutable variable replacement.
chatInputCompletions.test.ts	Tests completion ranking and filtering.
chatVariables.test.ts	Tests compact references and request reconstruction.
chatRequestParser.ts	Passes attachment metadata into parsed parts.
chatParserTypes.ts	Reconstructs full attachment entries.
common/attachments/chatVariables.ts	Defines compact attachment conversions.
chatVariableEntries.ts	Adds variable replacement helpers.
chatInputCompletionUtils.ts	Adds attachment completion utilities.
chatInputCompletions.ts	Provides prioritized attachment suggestions.
chatWidget.ts	Preserves attachments while applying prompt ranges.
browser/attachments/chatVariables.ts	Detects and applies existing references.
chatDynamicVariables.ts	Compacts references to attached context.
chatAccessibilityHelp.ts	Documents inline attachment references.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 2
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 17/17 changed files
• Comments generated: 3
• Review effort level: Medium

[Copilot]

Review details

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/attachments/chatDynamicVariables.ts:260

• The lifetime check occurs before another await. If the attachment is removed/replaced or this contribution is disposed while thumbnail.arrayBuffer() resolves, the continuation repopulates the cleared hover cache and calls updateDecorations() on a disposed editor. Perform the attachment/disposal check after the final await.

		const thumbnail = await getOrCreateImageThumbnail(attachment.id, buffer, CHAT_IMAGE_HOVER_THUMBNAIL_MAX_SIZE);
		if (!thumbnail || !this.widget.input.attachmentModel.attachments.includes(attachment)) {
			return;

• Files reviewed: 15/15 changed files
• Comments generated: 1
• Review effort level: Medium