Support explicit [\symbolName\](path/to/file.py) symbol links and deprecate optimistic backtick matching #313533

Summary

The chat response renderer currently supports "optimistic" symbol linkification: when the model outputs `symbolName`, the Copilot extension searches the workspace for a symbol with that name and—if found—emits a ChatResponseAnchorPart that renders as a pill widget. This approach is prone to false positives (linking to wrong files, unrelated variables, etc.) which degrades user trust in the links feature.

We want to shift to a more precise model: the model explicitly outputs [`symbolName`](path/to/symbol/file.py) and VS Code renders that as a proper inline anchor (pill) widget rather than a plain hyperlink.

Problem with the current approach

• The workspace symbol search triggered by bare `symbolName` can match completely unrelated symbols — e.g., a local variable named list gets linked when the model meant the abstract concept of a list.
• False-positive links erode user trust; users stop clicking or stop reading the linked pills altogether.
• The optimistic matching is a workaround for models that don't reliably output full path links. With model improvements and prompting, this workaround is no longer necessary and is actively harmful.

VS Code should render myFunction as a clickable pill widget (same as today's InlineAnchorWidget) pointing to src/utils/helpers.py, rather than a plain blue hyperlink.

References

• chatInlineAnchorWidget.ts — renderFileWidgets, InlineAnchorWidget
• annotations.ts — annotateSpecialMarkdownContent, contentRefUrl
• chatMarkdownDecorationsRenderer.ts — walkTreeAndAnnotateReferenceLinks