Unify GSC analytics response contract and status-driven consumers

[AJaySi]

Motivation

• Ensure get_search_analytics always returns a single, predictable shape so downstream code can rely on keys rather than presence checks.
• Replace ad-hoc early returns (auth failure, not connected, no data, verification/API errors, exceptions) with structured responses that include a status marker and empty collections.
• Normalize cached/legacy payloads so previously cached responses conform to the new contract.
• Simplify and harden consumers (handler, scheduler, onboarding normalizers) to use status + guaranteed keys instead of brittle key-existence logic.

Description

• Added helper builders inside GSCService.get_search_analytics: _base_payload(status, error) and _normalize_payload(payload) that guarantee the unified contract with keys: overall_metrics.rows, query_data.rows, page_data.rows, query_page_data.rows, verification_data.rows, date_range (start/end), site_url, startDate/endDate/siteUrl aliases, status, and optional error/warning.
• Refactored all early-return branches in get_search_analytics to return the canonical payload with appropriate status values (examples: not_connected, auth_failed, no_data, verification_failed, api_error, partial_success, success, error) and empty arrays instead of ad-hoc dict shapes.
• Normalized successful and partial-success responses to include canonical date_range, site_url, and status and ensure cached responses are converted via _normalize_payload before being returned.
• Updated consumers to rely on status and guaranteed keys: backend/services/analytics/handlers/gsc_handler.py now derives metrics from overall_metrics/query_data and computes connection status from status; backend/services/scheduler/executors/gsc_insights_executor.py checks status rather than presence of 'error'; onboarding normalizer normalize_gsc_analytics gates processing based on status.

Testing

• Ran python -m compileall backend/services/gsc_service.py backend/services/analytics/handlers/gsc_handler.py backend/services/scheduler/executors/gsc_insights_executor.py backend/api/content_planning/services/content_strategy/autofill/normalizers/analytics_normalizer.py and all modified modules compiled without syntax errors.
• No automated unit tests were added or modified as part of this change.

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Codex Task