Summary

Fixes flaky test test_sender_level_gauges_cleanup_on_post_stop by using a unique sender ID for test isolation.

Problem

The test was non-deterministic because:

• It used a shared sender ID (SENDER.1) for Prometheus metrics
• Tests run in parallel
• Parallel tests using the same sender label interfered with each other's metric values

Test A                          Test B (parallel)
------                          ----------------
1. Set ESCROW_BALANCE = 1000    
2. Stop sender account          1. Create sender account
3. Expect ESCROW_BALANCE = 0    2. Set MAX_FEE_PER_SENDER = 600
   ^                               ^
   FAILS: sees 600 from Test B

Solution

• Add optional sender_id parameter to create_sender_account test helper
• Return sender_id from create_sender_account for tests to use
• Update the flaky test to use Address::random() for test isolation
• Update all call sites to handle the new 6-element return tuple

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