Apply MPP receive timeout to keysend payments

[tnull]

Fixes #4553.

Incomplete keysend MPPs skipped the receive timeout path, allowing partial payments to hold HTLC slots until CLTV expiry instead of failing after MPP_TIMEOUT_TICKS. Apply the existing total_mpp_amount_msat completeness check to all MPP receives and add a regression test covering the keysend case.

The timeout logic was originally added only for
invoice-backed MPPs in 2022, and that invoice-only guard remained when receive-side MPP keysend support landed in 2023, leaving this gap latent until now.

Co-Authored-By: HAL 9000

[ldk-reviews-bot]

👋 Thanks for assigning @carlaKC as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've thoroughly re-reviewed the entire diff, including:

1. The core change in channelmanager.rs (lines 8881-8900): Removal of the OnionPayload::Invoice guard so MPP timeout applies to all payment types.
2. Condition matching: total_mpp_value <= total_intended_recvd_value in timer_tick_occurred (line 8887) correctly mirrors total_intended_recvd_value >= total_mpp_value in process_pending_htlc_forwards (line 8412).
3. Single-path keysend safety: total_mpp_amount_msat falls back to outgoing_amt_msat for non-MPP keysend, so the completeness check passes immediately — no behavioral change for single-path keysend.
4. Test correctness: Verified send_spontaneous_payment API usage matches the signature, RecipientOnionFields::secret_only(payment_secret, 200_000) correctly sets the total MPP amount, pass_along_path uses expected_preimage = None for incomplete first path and Some(payment_preimage) for the completing second path, and PaymentParameters::for_keysend with true enables MPP.

No issues found.

[carlaKC]

lgtm - just one comment on the test

[TheBlueMatt]

thanks. @carlaKC should land when she likes the test.

[ThomsenDrake]

LGTM. The fix is minimal and correct — applying the total_mpp_amount_msat completeness check to all MPP receive paths closes the gap where partial keysend payments could leak HTLC slots.

The regression test in payment_tests.rs is a good coverage anchor. One question: does parse_recv_partial_keysend_payment have any similar paths that bypass the timeout check?

[tnull]

parse_recv_partial_keysend_payment

Could you expand on what your're referring to? This method doesn't seem to exist in the codebase?