feat: add calculate send all fee for onchain

[ben-kaufman]

Summary

• Add OnchainPayment::calculate_send_all_fee() as the fee-estimation counterpart of
  send_all_to_address(), allowing callers to preview the mining fee for a drain transaction
  before broadcasting.
• Supports retain_reserves flag to match send_all_to_address behavior (anchor channel
  reserve handling).
• Add integration tests validating estimated fee matches actual fee after broadcast.

Motivation

calculate_total_fee() fails when called with amount_sats equal to the total UTXO value,
because the internal ExactRetainingReserve validation adds a fee buffer that exceeds the
available value. The existing InsufficientFunds fallback in calculate_total_fee also fails
because it forwards the caller's specific utxos_to_spend, which conflicts with BDK's
drain_wallet() mode.

calculate_send_all_fee solves this by directly using AllRetainingReserve /
AllDrainingReserve with utxos_to_spend: None, matching how send_all_to_address builds
the actual transaction.

Integration

• release: v0.7.0-rc.30
• bitkit-android: synonymdev/bitkit-android@1c3bad5
• bitkit-ios: synonymdev/bitkit-ios@9b4e402

Test plan

• calculate_send_all_fee_matches_actual — funds a node, estimates fee, sends via
  send_all_to_address, asserts estimated fee equals actual fee
• onchain_send_all_retains_reserve — verifies retain vs drain fee estimation with open
  anchor channels
• Validates error cases: zero balance, wrong-network address
• rely on CI + e2e in bitkit-ios and bitkit-android

[ovitrif]

LGTM 👏🏻

Thanks for adding the tests, makes it much better to validate and learn about the setup at the same time.

Plus it proves older changes as well as the new changes work as expected.