refunder: Make gas price parameters configurable

[ahhda]

Problem

The refunder was experiencing issues with hardcoded gas price parameters that were too low for current network conditions:

Errors observed:

1. Refunding txs are likely not mined in time, as the current gas price 826120518137.4663 is higher than MAX_GAS_PRICE specified 800000000000
2. transaction gas price below minimum: gas tip cap 5773015156, minimum needed 25000000000

Root cause:

• MAX_GAS_PRICE was hardcoded to 800 Gwei (current gas prices ~826 Gwei)
• START_PRIORITY_FEE_TIP was hardcoded to 2 Gwei (node requires minimum 25 Gwei)

Solution

This PR makes both gas price parameters configurable via CLI arguments or environment variables:

• --max-gas-price / MAX_GAS_PRICE (default: 2000 Gwei / 2,000,000,000,000 wei)
• --start-priority-fee-tip / START_PRIORITY_FEE_TIP (default: 30 Gwei / 30,000,000,000 wei)

The new defaults are set higher to accommodate current network conditions while still being configurable for future adjustments.

Changes

• Added two new CLI arguments with environment variable support
• Refactored Submitter and RefundService to accept configurable parameters
• Updated tests to use test-specific constants
• Updated argument display formatting to show new parameters

Testing

• Code compiles successfully (cargo check -p refunder)
• No linter errors
• Unit tests updated and passing

[ahhda]

Note on E2E Test Failure

The CI failure in local_node_eth_flow_indexing_after_refund appears to be unrelated to the refunder gas price configuration changes:

Why it's unrelated:

• The test timeout occurs during order settlement (waiting for buy token balance), not during refund operations
• The refunder changes only affect gas price parameters used when submitting refund transactions
• There were recent changes to ethflow background indexing (PR Index ethflow orders in the background #3849) that may have affected test timing

Test failure details:

• Test: local_node_eth_flow_indexing_after_refund
• Location: crates/e2e/tests/e2e/ethflow.rs:522
• Error: Timeout (30s) waiting for order settlement

This may be a flaky test or a separate issue with ethflow indexing timing. The refunder-specific changes compile successfully and pass unit tests.