refactor(ads-client): housekeeping - sort fields/methods, add UniFFI defaults, split docs, add Client in HttpCache

[Almaju]

• Sort struct fields alphabetically (HttpCache)
• Sort methods: constructors → pub (alpha) → private (alpha) in HttpCache, AdsClient, MARSClient
• Add #[uniffi(default = None)] to optional FFI fields for better consumer ergonomics (iab_content, cache config fields, ttl_seconds)
• Replace manual Default impl with #[derive(Default)] (clippy)
• Split usage.md into per-language files (JavaScript, Kotlin, Swift) with only consumer-facing examples; update README links
• Add viaduct::Client in HttpCache to make it compatible with future OHTTP impl

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• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
  • Note:
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• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due diligence applied in selecting them.

[copyrighthero]

Code review

Found 1 issue:

1. Integration test compile error -- send_with_policy signature changed to require a new client: &Client first argument, but all three calls in the integration test file were not updated. The unit tests in src/http_cache.rs were correctly updated (they create a make_client() helper), but the integration test calls still pass only 2 arguments, causing E0061 ("this method takes 3 arguments but 2 arguments were supplied").

application-services/components/ads-client/integration-tests/tests/http_cache.rs

Lines 58 to 70 in 94aadf8

	.expect(1)
	.create();
	// First call: miss -> store
	let (_, outcomes) = cache
	.send_with_policy(
	req.clone(),
	&CachePolicy::CacheFirst {
	ttl: Some(Duration::from_secs(test_ttl)),
	},
	)
	.unwrap();
	assert!(matches!(outcomes.last().unwrap(), CacheOutcome::MissStored));

application-services/components/ads-client/integration-tests/tests/http_cache.rs

Lines 72 to 76 in 94aadf8

	// Second call: hit (no extra HTTP due to expect(1))
	let (response, outcomes) = cache
	.send_with_policy(req.clone(), &CachePolicy::default())
	.unwrap();
	assert!(matches!(outcomes.last().unwrap(), CacheOutcome::Hit));

application-services/components/ads-client/integration-tests/tests/http_cache.rs

Lines 87 to 91 in 94aadf8

	std::thread::sleep(Duration::from_secs(test_ttl));
	let (response, outcomes) = cache
	.send_with_policy(req, &CachePolicy::default())
	.unwrap();
	assert!(matches!(outcomes.last().unwrap(), CacheOutcome::MissStored));

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[Almaju]

Thanks @copyrighthero
I pushed the fix for the integration test, CI should go green now.