fix(cql2-rewrite-links): preserve client filter from POST body

[nicolaracco]

Cql2RewriteLinksFilterMiddleware strips filter and filter-lang from the body of paginated next links when the client originally sent the filter inside a POST /search body. Following the next link therefore loses the filter, broadening every page after the first.

The middleware was reading the client's original filter only from request.query_params, which is empty for POST search. With nothing to restore, it fell into the strip branch and popped the fields outright.

Reproduction

curl -X POST https://<proxy>/search \
  -H 'content-type: application/json' \
  -d '{
    "collections": ["my-collection"],
    "limit": 10,
    "filter": {"op":"=","args":[{"property":"processing:version"},"A01"]},
    "filter-lang": "cql2-json"
  }'

next.body returned by the proxy:

{ "collections": ["my-collection"], "limit": 10, "token": "..." }

vs the same call against the upstream STAC API directly, which returns:

{
  "collections": ["my-collection"],
  "limit": 10,
  "token": "...",
  "filter": {"op":"=","args":[{"property":"processing:version"},"A01"]},
  "filter-lang": "cql2-json"
}

Fix

Symmetric with the existing query-string read:

1. For any POST/PUT/PATCH (no path scoping — the JSON-decode no-ops if the body is absent or unparseable, mirroring how query_params.get("filter") no-ops when the key is missing), drain the request body, JSON-decode it, and capture body["filter"] and body["filter-lang"] using a sentinel so we can distinguish "client sent no filter" from "client sent some value".
2. Replay the buffered bytes via a wrapped receive so Cql2ApplyFilterBodyMiddleware and the upstream still see the unmodified body.
3. When rewriting each link.body, prefer the captured client values over the existing query-string-derived behavior; fall back to the prior pop-when-stripping-only path otherwise.

The query-string href branch is unchanged.