fix: don't forward stale Content-Encoding/Content-Length to the client

[IntenZe1337]

Problem

Non-streaming responses are returned with the upstream headers copied verbatim:

return Response(content=response.content, status_code=..., headers=dict(response.headers))

httpx transparently decompresses the response body, so by the time we build that
Response the content is already plaintext. But dict(response.headers) still carries
the upstream Content-Encoding: gzip and the compressed Content-Length — headers
that describe a body which no longer exists.

Any client that sends Accept-Encoding: gzip then tries to gunzip plaintext and fails.

Reproduction

$ curl --compressed -sS http://localhost:8082/v1/messages \
    -H 'content-type: application/json' \
    -H 'x-api-key: dummy' \
    -H 'anthropic-version: 2023-06-01' \
    -d '{"model":"claude-3-5-haiku-20241022","max_tokens":16,
         "messages":[{"role":"user","content":"hi"}]}'
curl: (61) Error while processing content unencoding: invalid stored block lengths

CURLE_BAD_CONTENT_ENCODING, exit code 61. Without --compressed the same request
succeeds, which is what makes this easy to miss during manual testing — but most real
HTTP clients negotiate gzip by default.

Fix

Strip the transport-describing / hop-by-hop headers before forwarding and let the ASGI
server compute correct values for the body actually sent:

HOP_BY_HOP_HEADERS = frozenset({
    "content-encoding", "content-length", "transfer-encoding", "connection", "keep-alive",
})

def passthrough_headers(headers) -> dict:
    return {k: v for k, v in headers.items() if k.lower() not in HOP_BY_HOP_HEADERS}

Applied to the two non-streaming return paths: /v1/messages and
/v1/messages/count_tokens. Streaming responses build their headers explicitly and were
never affected.

connection, keep-alive and transfer-encoding are included because they are
per-connection headers that must not be relayed across a proxy hop (RFC 9110 §7.6.1);
they were not the cause of the observed failure but belong in the same set.

Scope

20 lines added, 2 changed, proxy.py only. No behaviour change for clients that do not
request compression.