feat(ads-client): support content categories on ad requests (AC-109)

[Almaju]

Summary

Implements AC-109: support ad requests with content categories on HNT.

• Adds a request-level flags.contextual_placement flag, exposed via MozAdsRequestOptions.contextualPlacement (default false).
• Per-placement IAB content (taxonomy + category IDs) was already wired; the outgoing JSON now omits content when unset to match the schema in the ticket exactly.
• The flag is included in the cache hash only when set, so non-contextual callers keep their existing cache entries.

Wire format produced when a caller passes both:

{
  "context_id": "…",
  "flags": {"contextual_placement": true},
  "placements": [
    {"placement": "newtab_stories_v2_1", "count": 1},
    {"placement": "newtab_stories_v2_3", "count": 1, "content": {"taxonomy": "IAB-3.0", "categories": ["338"]}}
  ]
}

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes — MozAdsRequestOptions.contextualPlacement has a UniFFI default of false, so existing callers compile and behave as before.
• Quality: This PR builds and tests run cleanly (cargo test -p ads-client, cargo clippy -p ads-client --all-targets -- -D warnings, cargo fmt --check).
• Tests: This PR includes thorough tests
  • Unit tests covering: flag-on serialization matching the AC-109 example, flag-off omits flags, mixed-content placements, flag participates in the cache hash.
  • Added a manual integration contract test (test_contract_image_with_categories_prod) that hits the prod MARS endpoint with iabContent + contextualPlacement: true. Run with cargo test -p ads-client-integration-tests -- --ignored.
• Changelog: CHANGELOG.md entry added under v153.0 → Ads Client.
• Dependencies: No new dependencies.