feat: add Bing and Google web search providers for AI discovery

[linroid]

Summary

• Add BingSearchProvider (Bing Web Search API v7) and GoogleSearchProvider (Google Custom Search JSON API) as real implementations of SearchProvider
• Auto-resolve provider from SearchConfig.provider in AiModule.create() — no manual wiring needed
• CLI, Desktop, and Android read env vars (BING_SEARCH_API_KEY or GOOGLE_SEARCH_API_KEY + GOOGLE_SEARCH_CX) to auto-select the provider
• DummySearchProvider remains the fallback when no search API is configured

Test plan

• Unit tests for query building (buildQuery, buildMultiSiteQuery) with 0, 1, N sites
• MockEngine integration tests for JSON parsing, empty results, missing fields, HTTP errors
• ./gradlew :ai:discover:test passes
• CLI, Desktop, Android modules compile clean
• Manual: BING_SEARCH_API_KEY=<key> OPENAI_API_KEY=<key> ./gradlew :cli:run --args="ai-discover 'ubuntu 24.04 iso'"
• Manual: GOOGLE_SEARCH_API_KEY=<key> GOOGLE_SEARCH_CX=<cx> OPENAI_API_KEY=<key> ./gradlew :cli:run --args="ai-discover 'ffmpeg download'"
• Verify no search env vars → DummySearchProvider (empty results, no crash)

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[github-actions]

Test Results

595 tests   - 350   595 ✅  - 350   7s ⏱️ -5s
 50 suites  -  32     0 💤 ±  0 
 50 files    -  32     0 ❌ ±  0 

Results for commit 3ea7c8f. ± Comparison against base commit 30b0e51.

This pull request removes 350 tests.

wasmJsBrowserTest.com.linroid.ketch.api.DownloadPriorityTest ‑ ordinal_ordering[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ isComplete_whenFullyDownloaded[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ isComplete_whenOverDownloaded[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ isNotComplete_whenPartial[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ isNotComplete_whenZeroTotal[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ percent_complete[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ percent_halfComplete[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ percent_zeroProgress[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadProgressTest ‑ percent_zeroTotal_returnsZero[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
wasmJsBrowserTest.com.linroid.ketch.api.DownloadRequestTest ‑ blankUrl_throws[wasmJs, browser, ChromeHeadless144.0.0.0, Linux0.0.0]
…