feat(copilot): opt-in HTTP cache for the Node fetch fetcher (#317721)

* feat(copilot): opt-in HTTP cache for the Node fetch fetcher

Adds an opportunistic cache support to the Copilot Node fetch path. The cache
is strictly opt-in per request and composes with the existing VSCode proxy
and CA-injection patch.

- `__vscodeCreateFetchPatch({ interceptors })` lets the extension host
  build a second proxy-aware `fetch` with extra undici interceptors. The
  default `__vscodePatchedFetch` is unchanged.
- `NodeFetchFetcher` builds an undici cache interceptor once at
  construction time and uses the factory to produce a `cachedFetch` that
  routes through both the proxy patch and the cache. Requests are tagged
  with an internal `__copilotCachePatch` marker (stripped before fetch);
  unmarked requests keep going through the regular patched fetch. When
  the host lacks the factory, caching is silently disabled so requests
  never bypass the proxy patch.
- `FetchOptions.cache?: boolean` — opportunistic hint. Fetchers without
  cache support ignore it; fallback to other fetchers is unaffected.
- `Response.cacheStatus` and `FetchTelemetryEvent.cacheStatus`:
  `'hit' | 'stale-hit' | 'revalidated' | 'miss' | 'bypass'`.
- New setting `github.copilot.advanced.debug.nodeFetchCache`:
  `'off' | 'memory' | 'persistent'` (default `'memory'`). `'persistent'`
  uses undici's SQLite store under the extension's global storage
  (`undici-cache.v1.sqlite`) when available, otherwise falls back to
  memory.
- New `taggedCacheInterceptor` wraps `undici.interceptors.cache` and
  stamps a private `VSCODE_CACHE_STATUS_HEADER` on the response so the
  base fetcher can read the outcome without parsing undici internals.
- `BaseFetchFetcher` exposes an overridable `_buildRequestInit` hook and
  reports `cacheStatus` on `Response` and `fetchTelemetry`.

Notes
- No behavior change for callers that don't set `cache: true`.
- The cache interceptor is constructed once per fetcher instance; the
  composed dispatcher chain is reused so connection pooling is preserved.
- Depends on https://github.com/microsoft/vscode-proxy-agent/pull/100

For https://github.com/microsoft/vscode/issues/308310

* fix: undici integration tests

- drop the `age` header gate from classify(): undici's cache interceptor
only adds if-modified-since / if-none-match when revalidating a stored
entry, so `state.conditional` alone is a sufficient signal. The age header
is not guaranteed on a revalidated 200, which caused 'revalidated' to be
misreported as 'miss'.

- the etag integration test used
`cache-control: max-age=0, must-revalidate`, which undici treats as
already-stale on arrival and refuses to store (cache-handler.js bails when
`now >= absoluteStaleAt`), so there was nothing to revalidate on the second
call. Switch the origin to `public, max-age=60` and pass
`cache-control: no-cache` on the second request to drive undici's
needsRevalidation() path, which dispatches with if-none-match and serves
the cached body on 304.

* chore: fix lint

* chore: cleanup cache marker in favor of function arg