feat(ads-client): add OHTTP support (AC-94)

[Almaju]

Summary

• Add ohttp: bool field to MozAdsRequestOptions and new MozAdsCallbackOptions { ohttp: bool } to opt individual requests into OHTTP routing
• Add OHTTP support to all MARS request paths: request_tile_ads, request_image_ads, request_spoc_ads, record_impression, record_click, report_ad
• All new options parameters use UniFFI defaults so existing callers require no changes
• Move OHTTP_CHANNEL_ID from http_cache into client — http_cache is now generic and has no ads-client knowledge
• Add #[uniffi(default = None/false)] to optional fields across MozAdsPlacementRequest, MozAdsCacheConfig, MozAdsRequestCachePolicy for better ergonomics in consumer code
• Split docs/usage.md into per-language files (usage-javascript.md, usage-kotlin.md, usage-swift.md) with correct language-specific examples
• Add NSS build steps to CI workflow (required since viaduct's ohttp feature depends on NSS)
• Add integration test test_contract_tile_ohttp_prod against production MARS over OHTTP

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, or
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• Tests: This PR includes thorough tests or an explanation of why it does not
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  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
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