fix(ext/fetch): raise default HTTP/2 SETTINGS_MAX_HEADER_LIST_SIZE to 256KB - #36558

Fixes #36462.

Problem

fetch()ing certain HTTP/2 responses fails with TypeError: fetch failed
(stream error detected: unspecific protocol error detected) while the same
request works with curl and Node.

Root cause

hyper defaults the HTTP/2 client's SETTINGS_MAX_HEADER_LIST_SIZE to 16KB.
When a server sends a response with a large header block (e.g. a long
content-security-policy header, or many set-cookie / x-* headers), the
decoded header list exceeds 16KB and the h2 layer resets the stream with a
PROTOCOL_ERROR before the response ever reaches JavaScript.

Confirmed with h2 tracing on the reproducer from the issue:

h2::frame::headers  - load_hpack; header list size over max
h2::proto::streams::recv - stream error REQUEST_HEADER_FIELDS_TOO_LARGE -- frame is over size
send_reset reason=PROTOCOL_ERROR, initiator=Library

Browsers and curl advertise much larger limits, so these responses work
everywhere except Deno.

Fix

Default the client SETTINGS_MAX_HEADER_LIST_SIZE to 256KB (the value
Chrome advertises) instead of hyper's 16KB, so fetch() interoperates with the
same servers browsers do. Users can still override it via
Deno.createHttpClient({ http2MaxHeaderListSize }).

Tests

• Enlarged the existing /large_headers test-server route to ~29KB decoded
  (over the old 16KB default, under the 64KB the existing "large enough" test
  uses).
• Added fetchHttp2LargeHeadersDefault regression test: a default client
  fetching a large-header response now succeeds.

Verified the reproducer URL now returns 200, and that the old binary rejects
the enlarged route while the patched binary accepts it.