Fix subscribe:true flag for PUT operations

[sanity]

Summary

• Fixed subscription logic in PUT operations to properly subscribe the initiating peer
• Added comprehensive unit test to verify UPDATE operations work after PUT with subscribe:true
• Resolves River chat UPDATE failures that occurred after creating rooms

Problem

The subscribe: true flag on PUT operations was not properly subscribing the initiating peer to the contract. This caused UPDATE operations to fail with "missing contract" errors, particularly affecting River chat functionality.

Solution

1. Fixed subscription logic: Removed the is_seeding_contract check that was preventing subscription
2. Added contract verification: Verify the contract is queryable locally before subscribing
3. Proper subscription request: Use try_get=true to handle cases where contract isn't fully available
4. Local subscriber registration: Ensure the peer is registered as a local subscriber

Testing

• Added comprehensive unit test test_put_subscribe_enables_update that verifies:
  • PUT with subscribe:true properly subscribes the initiating peer
  • Subsequent UPDATE operations succeed without requiring separate SUBSCRIBE
• Manually tested with River chat:
  • Created room successfully
  • Sent messages (UPDATE operations) successfully
  • Messages persist and can be retrieved

Test Results

✓ Room creation successful
✓ Message sending (UPDATE) successful  
✓ Messages retrieved correctly

Fixes #1765

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