Fix cluster_api_url("mainnet") raising AttributeError

[Amadeus-22]

Problem

Since 0.40.2, cluster_api_url("mainnet") fails:

>>> from solana.utils.cluster import cluster_api_url
>>> cluster_api_url("mainnet")
AttributeError: 'ClusterUrls' object has no attribute 'mainnet'

#699 renamed the cluster literal to "mainnet", but the function still resolves the URL with getattr(urls, cluster), and the ClusterUrls field is still mainnet_beta. "mainnet" is the only value the type allows for that cluster, so there is no way to get the mainnet URL from this helper.

Change

cluster_api_url maps "mainnet" to the mainnet_beta field. I left ClusterUrls as it is so the public model does not change; renaming the field would be the alternative if you prefer it, and I can switch to that.

Tests

tests/unit/test_cluster_api_url.py only covered devnet. It now also checks testnet, and a new test_mainnet covers "mainnet" with and without TLS. test_mainnet fails on master with the error above and passes with the change.

pytest tests/unit passes (137 tests); ruff format --check, ruff check and mypy are clean on the touched files. Added a Fixed entry under Unreleased in the changelog.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.31%. Comparing base (fa1f4c9) to head (ff0c6df).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #707   +/-   ##
=======================================
  Coverage   95.30%   95.31%           
=======================================
  Files          25       25           
  Lines        1917     1919    +2     
=======================================
+ Hits         1827     1829    +2     
  Misses         90       90           

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