fs/kvdb:extend val_len to 2 bytes to support property value up to 1024

[wjh199742]

Encode val_len as 2 little-endian bytes in the client/server wire protocol and raise PROP_VALUE_MAX from 255 to 1024, so larger property values can be transferred.

Note: Please adhere to Contributing Guidelines.

Summary

Update KVDB client/server protocol to support larger property values.

The original protocol uses 1-byte val_len, which limits property values
to 255 bytes. This patch changes val_len to 2-byte little-endian format
and increases PROP_VALUE_MAX from 255 to 1024.

This allows KVDB to transfer larger property values through the
client/server interface.

Impact

• Increase maximum property value size from 255 to 1024 bytes.
• Client and server protocol format are changed, so both sides need to
  be updated together.
• No impact for existing property values smaller than 255 bytes.

Testing

Tested KVDB client/server communication with:

• Property value size < 255 bytes
• Property value size > 255 bytes
• Property value size close to 1024 bytes

Verified that property set/get operations work correctly.

[github-actions]

❌ CLA Signature Required

@wjh199742 Some contributors need to sign the CLA:

• jinghengwan@bestechnic.com ❌ Needs to sign CLA

Please:

1. Sign the CLA at: https://www.openvela.com/#/community/cla
2. After signing, comment /check-cla to recheck

📋 View detailed check results: Action Run #29067966506

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💡 Tip: All contributors must sign the CLA before the PR can be merged.

[wjh199742]

Hi @xiaoxiang781216 ,

Could you please help review this PR?

Thanks.