Fix #2040: Update Balance immature calculation for MTP-based maturity

[kallal79]

Description

This PR fixes #2040 by updating the Balance immature category calculation to consider Median Time Past (MTP) based maturity for time-locked transactions.

Previously, Balance.immature only tracked coinbase outputs that hadn't reached 100 block confirmations, ignoring absolute time-locks (nLockTime). With this change, time-locked transactions (those with absolute nLockTime >= 500,000,000) are also properly classified as immature until the chain tip's MTP exceeds their locktime.

Changes made:

• Added lock_time: LockTime to the FullTxOut struct in crates/chain/src/chain_data.rs.
• Implemented is_mature_at_mtp() and is_confirmed_and_spendable_at_mtp() on FullTxOut to handle time-based locktime checks against an optional mtp parameter.
• Updated the signature of CanonicalView::balance() to safely accept an mtp: Option<u32> and delegate to the new MTP maturity checks.
• Updated the Balance documentation in crates/chain/src/balance.rs to reflect the updated classification definition.
• Modified existing tests to supply None for MTP in older benchmarks/tests, maintaining compatibility.
• Added a robust unit test test_balance_mtp_maturity in test_indexed_tx_graph.rs to explicitly verify time-locked outputs accurately shift from immature to confirmed as the MTP increases past the locktime.

Notes to the reviewers

Because MTP isn't always available (e.g. missing block timestamps in some local state), the new balance() parameter uses Option<u32>. If None is provided, time-locked transactions are conservatively classified as immature to avoid falsely showing them as confirmed and spendable.

Changelog notice

• Fixed Balance immature tracking to accurately calculate maturity for transactions utilizing absolute MTP-based time-locks (Issue Update Balance immature category calculation to consider MTP-based maturity #2040).

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR