feat: convert merge mining cucumber tests to RxT

[0xskr33p]

Description

Convert merge mining integration tests from RandomX-Monero (RxM) to RandomX-Tari (RxT) by switching from the standalone minotari_merge_mining_proxy to the base node's built-in xmrig_proxy.

Key changes:

• port_pool.rs: Added xmrig_proxy port to BaseNodePorts; renamed merge_mining_proxy_ports -> xmrig_proxy_ports; removed standalone proxy port allocation methods.
• base_node_process.rs: Enabled built-in xmrig_proxy during base node spawn (xmrig_proxy_enabled, xmrig_proxy_address, xmrig_proxy_wallet_payment_address).
• merge_mining_proxy.rs: Rewrote from external process launcher to lightweight HTTP client targeting the base node's built-in proxy; removed all Monero-specific config (monerod_url, wallet gRPC); removed unsupported methods (get_last_block_header, get_block_header_by_hash).
• merge_mining_steps.rs: Updated step definitions for new RxT API response format (76-byte hex blobs, submitblock instead of submit_block, count instead of height).
• MergeMining.feature: Removed @broken tag; removed Monero origin submission scenario; adapted validation steps.
• BlockExplorerGRPC.feature: Removed wallet dependency that was only needed for the old standalone proxy.
• Cargo.toml: Removed minotari_merge_mining_proxy dependency.
• world.rs: Updated after hook to return xmrig_proxy ports to pool on base node shutdown.

Motivation and Context

#7737

How Has This Been Tested?

cargo test --test cucumber --package tari_integration_tests -- --tags "@merge-mining"

Results:

• 2 scenarios passed (Merge Mining Proxy API Test, Simple Merge Mining)
• 12 steps passed
• 0 failures

What process can a PR reviewer use to test or verify this change?

1. Run the merge mining integration tests:

   cargo test --test cucumber --package tari_integration_tests -- --tags "@merge-mining"

2. Verify both scenarios pass without any external Monero daemon running – the tests should be fully self-contained.
3. Confirm no monerod connections are attempted (no network calls to Monero stagenet nodes).

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify

[gemini-code-assist]

Code Review

This pull request replaces the standalone merge mining proxy process with a built-in XMRig proxy in the base node for integration tests. Key changes include the removal of the minotari_merge_mining_proxy dependency, the addition of xmrig_proxy_port to the base node configuration, and a significant refactoring of MergeMiningProxyProcess into a lightweight HTTP client. Corresponding updates were made to the port pool management, test world cleanup, and Gherkin feature files to reflect the simplified architecture. I have no feedback to provide.