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]

I've assigned @jkczyz 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 reviewed the entire diff thoroughly, tracing through:

1. The core change in channelmanager.rs: Removal of the if let OnionPayload::Invoice { .. } guard so the MPP completeness check and timeout logic applies to both invoice and keysend payments.

2. Condition matching: Verified the completeness condition total_mpp_value <= total_intended_recvd_value in timer_tick_occurred is equivalent to total_intended_recvd_value >= total_mpp_value in process_pending_htlc_forwards (line 8412). These correctly stay in sync.

3. Single-path keysend safety: For non-MPP keysend (no payment_data), total_mpp_amount_msat falls back to outgoing_amt_msat (onion_payment.rs:282-285), which equals sender_intended_value, so the completeness check passes immediately and no timeout is triggered. Existing single-path keysend behavior is unaffected.

4. The test: Follows the established pattern from do_mpp_receive_timeout(true) exactly — same diamond topology, same partial relay, same fail-back verification. The RecipientOnionFields::secret_only(payment_secret, 200_000) correctly sets the total MPP amount matching the two-path route.

5. Keysend payment secret handling: Confirmed that has_recipient_created_payment_secret is false for keysend (onion_payment.rs:296), so the secret from get_payment_preimage_hash is used only for MPP correlation, not inbound payment verification.

No issues found.