Uniffi API mismatach followup

[rmn-boiko]

• Added full UniFFI SDK surface for RLN (UDL, UniFFI API layer, SDK module, exposed types, and lifecycle state bridge).
• Added UniFFI binding infrastructure and automation (build.rs, UniFFI configs, bindgen helper crate, generation scripts for Python/Kotlin/Swift/Android, CI artifact workflow).
• Added SDK-facing documentation and Python interop example covering node init/unlock/funding, UTXO creation, asset issuance, channel open, invoice creation, payment send, and status checks.
• Introduced supporting library/test structure updates to run SDK logic as a library and validate UniFFI behavior.
• Aligned UniFFI payment model with backend by adding preimage propagation end-to-end (SDK data -> UniFFI mapping -> exported type -> UDL).
• Expanded UniFFI API error mapping to cover a broader set of backend errors with consistent RlnError categories (NotFound / Conflict / InvalidRequest / Internal).
• Added tests validating the updated error mapping behavior and payment preimage mapping correctness.

[MaiborodaY]

Retested locally after merging the PR.

Confirmed fixed:

• Kotlin openchannel_optional_addr_forward now passes
• Kotlin openchannel_optional_addr_reverse now passes
• Python openchannel_fail_no_utxos now passes
• lib_sdk::payment::success passes with Payment.preimage validated as well

So the previously found UniFFI error-mapping gaps are now resolved on my side, and the Payment.preimage follow-up also looks good.

[txalkan]

Hi Roman, pls remember to add PR descriptions.

We can approve this PR after #9 is merged and the merge conflicts are resolved. Thank you

[gofman8]

@rmn-boiko let's merge