Add short-lived cache for documents and filter by language in linkifier.

[DanielFabian]

Fix: Optimize inline code symbol linkifier to avoid redundant file I/O

The inline code linkifier (InlineCodeSymbolLinkifier) was causing severe performance issues during LLM response streaming. For each backticked symbol (e.g., TextDocument), it would re-read every reference file from disk, resulting in O(N×M) file reads where N = symbol count and M = reference count.

Changes:

• Language gating: Skip files entirely when tree-sitter doesn't support their language (.log, .md, .json, etc.) in symbolMatchesOnly mode—zero I/O for unsupported files.

• Document cache: Add a per-resolver documentCache so each file is read at most once per response, reducing file reads from O(N×M) to O(M).

Impact: Responses with many backticked symbols and large context files (especially non-code files like logs) should no longer hang the UI. Should help with microsoft/vscode#275716

[Copilot]

Pull request overview

This PR optimizes the inline code symbol linkifier to address severe performance issues during LLM response streaming. The linkifier was performing O(N×M) file reads (N = backticked symbols, M = reference files) by re-reading every file for each symbol. The PR implements two key optimizations:

• Language gating: Skip files entirely when their language isn't supported by tree-sitter in symbolMatchesOnly mode
• Document caching: Add per-resolver document cache to ensure each file is read at most once per response

[Copilot]

[nitpick] Consider adding a JSDoc comment to document the purpose of the documentCache field. For example:

/** Cache of documents read during symbol resolution to avoid redundant file I/O within a single response. */
private readonly documentCache = new Map<string, Promise<SimpleTextDocument | undefined>>();

This would help future maintainers understand that the cache is per-resolver instance (and thus per-response) and is meant to avoid reading the same file multiple times during linkification of a single response.

Suggested change

	private readonly cache = new Map<string, Promise<vscode.Location[] | undefined>>();
	private readonly cache = new Map<string, Promise<vscode.Location[] | undefined>>();
	/**
	* Cache of documents read during symbol resolution to avoid redundant file I/O within a single response.
	* This cache is per-resolver instance (and thus per-response) and is meant to avoid reading the same file
	* multiple times during linkification of a single response.
	*/