Support Multiple Electrs Backends with Failover

[adamkrellenstein]

Summary

The service previously relied on a single Blockstream API endpoint for UTXO lookups and pubkey searches, which is unreliable. This changes config.ELECTRS_URL (single string) to config.ELECTRS_URLS (list) with automatic failover on connection, timeout, or HTTP errors.

• --electrs-url can now be specified multiple times for failover
• Mainnet defaults to both blockstream.info/api and mempool.space/api
• Prints a startup warning when using default URLs (not recommended for production)
• Fixes a pre-existing if/if bug in initialise.py for testnet/testnet4 network selection

Checklist

• Double-check the spelling and grammar of all strings, code comments, etc.
• Double-check that all code is deterministic that needs to be
• Add tests to cover any new or revised logic
• Ensure that the test suite passes
• Update the project release notes (release-notes-v11.0.5.md)
• Update the project documentation, as appropriate, with a corresponding Pull Request in the Documentation repository

[codecov]

Codecov Report

❌ Patch coverage is 32.43243% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.85%. Comparing base (bfef947) to head (93b187e).

Files with missing lines	Patch %	Lines
...party-core/counterpartycore/lib/backend/electrs.py	0.00%	13 Missing ⚠️
...rparty-core/counterpartycore/lib/cli/initialise.py	35.71%	9 Missing ⚠️
...arty-core/counterpartycore/lib/backend/__init__.py	0.00%	2 Missing ⚠️
...ounterparty-core/counterpartycore/lib/cli/setup.py	50.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (bfef947) and HEAD (93b187e). Click for more details.

HEAD has 8 uploads less than BASE

Flag	BASE (bfef947)	HEAD (93b187e)
	15	7

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #3270       +/-   ##
============================================
- Coverage    88.13%   57.85%   -30.28%     
============================================
  Files          108      106        -2     
  Lines        14584    14550       -34     
============================================
- Hits         12853     8418     -4435     
- Misses        1731     6132     +4401     

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