fix: use decode instead of deprecated decodepay in CLN backends

[sharpone74]

Follow-up in the same spirit as #290 / #307 — this time the Transactions breakage comes from CLN retiring an RPC out from under us.

Both CLN backends decode payment requests via decodepay:

res = await self._send_request("decodepay", params)              # cln_jrpc.py
res = await _make_local_call("decodepay", f"bolt11={pay_req}")   # cln_grpc.py

CLN deprecated decodepay in favor of decode in v24.11, now rejects it by default (observed on v25.12.1: JSON-RPC -32601 'Command "decodepay" is deprecated'), and removes it entirely in v26.04 (ElementsProject/lightning#8850). Since decode_pay_request is called for every invoice in the payment history, list_all_tx() fails wholesale and /lightning/list-all-tx returns HTTP 500 on every poll — the Transactions view in the web UI is permanently broken against current CLN.

Fix

• Call decode instead of decodepay in both cln_jrpc.py and cln_grpc.py (the CLI fallback's keyword changes from bolt11= to string=). For bolt11 invoices decode returns a superset of decodepay's fields, so PaymentRequest.from_cln_json needs no changes.
• Guard the one behavioral difference: decode reports recognized-but-invalid strings as a normal result with valid=false + warning_* fields instead of an RPC error. Unhandled, that would resurface the fix: handle missing bolt11 in CLN list_payments for keysend/BOLT12 #307 failure mode (KeyError swallowed by @logger.catch → None → TypeError in list_all_tx), so both backends map valid=false to a clear 400 with the warnings as detail.

Reproduction

• RaspiBlitz v1.12.1 / Core Lightning v25.12.1
• Open the Transactions tab in the web UI → 500 on every 20 s poll:

  2026-07-20 12:10:19 | ❌ | cln_jrpc.py:490 | Command "decodepay" is deprecated
  2026-07-20 12:10:19 | ℹ️ | logging.py:90 | 127.0.0.1:48548 - "GET /api/lightning/list-all-tx?reversed=true HTTP/1.0" 500
  

• Verified on the same node that lightning-cli decode returns every field from_cln_json consumes; with the patch applied the endpoint returns 200 and the Transactions view loads.

Notes

• Tests: added regression tests pinning both backends to decode (so decodepay can't sneak back) plus the valid=false → 400 path, and updated the shell-safety test for the new argv.
• decode predates the v24.11 deprecation by years, so this does not raise the minimum supported CLN version.

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