docs: document missing and fix erroneous JSON-RPC error codes

[daywalker90]

Audit the errors sections of the RPC schemas against the error codes the handlers actually raise, and fix the discrepancies.

FUNDING_STILL_SYNCING_BITCOIN (304) is only raised by fundpsbt and utxopsbt (wallet/reservation.c), and is forwarded verbatim by every other command which can return it. Document it on fundpsbt, utxopsbt, txprepare, withdraw, multiwithdraw, upgradewallet, fundchannel and multifundchannel. Conversely, fundchannel_start and openchannel_init do not call either: when the node is not synced they delay via topology_add_sync_waiter() and retry, so drop their stale 304.

Also fix the remaining discrepancies:

• delpay documented 211 for a status mismatch, but that path returns 208 (PAY_NO_SUCH_PAYMENT); 211 is never raised by delpay, so replace it;
• document missing codes: createinvoice 907 (INVOICE_OFFER_INACTIVE), enableoffer 1007 (OFFER_USED_SINGLE_USE), getroutes 217 (PAY_USER_ERROR) and addpsbtoutput 302/309;
• add errors sections to splice_init, splice_update and splice_signed (351-356), disableoffer and disableinvoicerequest (1001), preapproveinvoice (213), preapprovekeysend (214) and recover (1600).

Changelog-None

Important

26.09 FREEZE August 5th: Non-bugfix PRs not ready by this date will wait for 26.12.

RC1 is scheduled on August 17th

The final release is scheduled for September 7th.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

[Andezion]

The three splice commands (splice_init, splice_update, splice_signed) got the most extensive new errors sections in this pr (351-356). Did the audit include the async failure paths in lightningd/channel_control.c, or only the synchronous raises in param_channel_for_splice? i ask because i found several more splice-specific codes that appear to reach the pending rpc command via splice_command_for_chan():

• SPLICE_BROADCAST_FAIL = 350 (check_utxo_block, line 564)
• SPLICE_FUNDING_LOW = 357 (handle_splice_funding_error, line 245)
• SPLICE_STATE_ERROR = 358 (handle_splice_state_error, line 275)
• SPLICE_LOW_FEE/SPLICE_HIGH_FEE = 359/360 (handle_splice_feerate_error, line 305)
• SPLICE_ABORT = 362 (handle_splice_abort, line 364)
• SPLICE_CHANNEL_ERROR = 363 (channel_control_errmsg, line 1740)

[daywalker90]

Good catch. I added those as well.