fix(provider): avoid confirmation height underflow

[questfever]

Motivation

PendingTransactionConfig::with_required_confirmations accepts any u64, including 0.

Most heartbeat paths compute the confirmation target block with block_height + confirmations - 1. This can underflow when confirmations == 0 and the transaction is observed at block 0, and it can also overflow for very large confirmation counts.

The new-block path already treats confirmations <= 1 as immediately confirmed, so the historical/already-confirmed paths should handle 0 consistently instead of panicking.

Solution

• centralize pending transaction confirmation target calculation
• use saturating arithmetic for block_height + confirmations - 1
• add regression tests for required_confirmations = 0 in already-confirmed and lookbehind paths

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes