feat(alibaba): fix SSL issues and improve error handling

[XWind18]

Summary

Fix SSL connection issues for Alibaba Coding Plan provider in mainland China region and improve error messaging when API Key mode is unavailable.
Problem:

• bailian-beijing-cs.aliyuncs.com and other *-cs.aliyuncs.com endpoints frequently fail SSL handshake in CN region
• API Key mode is unavailable for some CN accounts, but error messages were unclear
  Solution:

1. Use bailian.console.aliyun.com instead of RPC endpoints to bypass SSL issues
2. Add new apiKeyUnavailableInRegion error type with actionable guidance
3. Improve error handling logic to properly distinguish login required vs API Key unavailable scenarios
   Changed Files:

• AlibabaCodingPlanAPIRegion.swift - Endpoint URL fix
• AlibabaCodingPlanProviderDescriptor.swift - Error retry logic
• AlibabaCodingPlanUsageFetcher.swift - New error type and messaging

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Commands Run

# Build verification
swift build -c release
# Test verification
swift test --filter "AlibabaCodingPlan"
# Code formatting
swiftformat Sources Tests
swiftlint --strict
Test Result: ✅ 32/32 Alibaba-related tests passed
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Screenshots
No UI changes.
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Linked Issue / Reference
- Related to: #574 (Add Alibaba Coding Plan provider)
- Fixes CN region SSL handshake failures reported in production
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Checklist
- [x] Code follows SwiftFormat/SwiftLint rules
- [x] Tests pass (swift test)
- [x] Commit message follows project conventions
- [x] No new dependencies added
- [x] Error handling improved