[REX-3084] Add getRecommendationPage for the recommendations page endpoint

[TimDumol]

Adds recommendations.getRecommendationPage(pageId, parameters, userParameters, networkParameters) for GET /recommendations/v1/pages/{page_id} (API reference), for server-rendered integrations. A page returns every pod configured on it in one request, deduplicated across pods server-side.

What it does

• Shared parameters: itemIds, variationId, section, term, numResults, filters, filterMatchTypes, preFilterExpression, fmtOptions, hiddenFields, variationsMap, plus the usual userParameters (session, client, user, segments, origin referrer, user IP/agent headers) and the security token header.

• podOverrides: { [podId]: { numResults, filters, filterMatchTypes, preFilterExpression, fmtOptions, hiddenFields, variationsMap } } is sent in bracket notation, e.g. pod_overrides[complete_the_look][filters][color]=red. Each value uses the same wire format as the top-level param: pre_filter_expression and variations_map are JSON strings. An override replaces the page-wide value for that pod. Page-wide keys inside an override are rejected client-side.

• Tracking: per-pod result_id. Each pod's own result_id is stamped onto that pod's results. The top-level result_id identifies the page request and is not a tracking id: send response.pods[].result_id with each pod's recommendation view/click events.

• Like getRecommendations, the returned promise exposes requestUrl.

• Types: RecommendationPageParameters, RecommendationPagePodOverride, RecommendationPageResponse, RecommendationPagePod.

• fmt_options replace semantics: hiddenFields is sent inside fmt_options, and an override replaces the page-wide value wholesale. So an override that sets only hiddenFields drops the page-wide fmtOptions for that pod, and vice versa.

Back-compat

getRecommendations is unchanged. Its user query params and headers now come from shared helpers. I compared URL + headers from origin/master and this branch on the same inputs, and they are identical apart from _dt. No docs/ or version changes.

Testing

• npm run lint: no errors (1 pre-existing no-console warning in a catalog spec)
• npm run test:types (tsd): passes, with new page type tests
• New mocked specs (stubbed fetch, no network): 8 passing. They cover the URL/path, shared + user params and headers, bracket encoding of pod_overrides (array filters, num_results=0, JSON pre_filter_expression/variations_map, fmt_options + hiddenFields), per-pod result_id stamping (page id never used), requestUrl, and the rejections.
• cspell (**/*.js): clean
• Live tests are added in describe.skip('getRecommendationPage - live'). The page endpoint is not yet enabled on the test index, and the test index has no page configured (/recommendations/v1/pages/pdp_b2c returns 404).
• I did not run the existing live suite locally (no test API key); CI will.

Blocked / follow-ups

• Live tests need the page endpoint enabled on the test index and a page configured there.

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