fix: harden capabilities deserialization when sharing fields are missing

[joshtrichards]

Summary

Refactor CapabilitiesDeserializer to safely handle partial files_sharing capability payloads, especially when Share API or public sharing is disabled. Previously merged PR #2803 helped, but did not cover all edge cases.

Changes

• split capability parsing into smaller helper methods
• use api_enabled as a gate for share-specific parsing
• avoid assuming optional files_sharing fields are always present
• preserve model defaults when fields like default_permissions or public.password are missing
• rename/update the misleading sharing test
• add regression tests for:
  • Share API disabled responses
  • missing default_permissions
  • missing ederation
  • public sharing disabled without password fields

Why

The server can legitimately omit parts of files_sharing depending on configuration. The previous deserializer assumed those fields always existed, which could cause crashes during first-time account setup when Share API was disabled or when certain sharing options were turned off.

Result

The app now tolerates partial sharing capability responses and falls back to the existing defaults in Capabilities instead of crashing.

Closes #2815
Closes #3165

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🏁 Checklist

• 🧪 Tests written, or not not needed

[codacy-production]

Not up to standards ⛔

🔴 Issues 1 medium

Alerts:
⚠ 1 issue (≤ 0 issues of at least minor severity)

Results:
1 new issue

Category	Results
Complexity	1 medium

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🟢 Metrics 17 complexity

Metric	Results
Complexity	17

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[github-actions]

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