fix: read SAFE_TRANSACTION_SERVICE_API_KEY when api_key is not provided

[Amine E. (aelmanaa)]

Fixes #2764

TransactionServiceApi.__init__ read SAFE_TRANSACTION_SERVICE_API_KEY as a default argument, so it was evaluated once at import time, and SafeBaseAPI.from_ethereum_client always passes api_key=None, which dropped it entirely. This makes api_key default to None and reads the variable when the instance is created if no key is given, so both the constructor and from_ethereum_client pick it up. An explicitly passed api_key still wins, and the signature is unchanged for callers.

Tests: TestTransactionServiceApiKey covers the key from the environment (constructor and from_ethereum_client, including the Authorization header), an explicit key overriding the environment, and no key. It runs without a Transaction Service API key or network, unlike TestTransactionServiceAPI, which skips when SAFE_TRANSACTION_SERVICE_API_KEY is unset.

Verification (Python 3.13, docker compose up -d db ganache):

• test_api_key_from_environment fails on main (AssertionError: None != 'env-api-key') and passes with this change; all three new tests pass.
• The reproduction from TransactionServiceApi.from_ethereum_client ignores SAFE_TRANSACTION_SERVICE_API_KEY #2764 now prints key-set-before-import Bearer key-set-before-import and key-set-after-import.
• pytest -m "not network": 519 passed, 0 failed. pre-commit run --all-files passes.

This was written with Claude Code; I reviewed and tested it.

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[Amine E. (aelmanaa)]

Before signing: the CLA link in the bot comment and in CONTRIBUTING (https://safe.global/cla) currently returns 404 (also www.safe.global/cla and /legal/cla), so I can't review the agreement. Could you share where the current CLA text lives? I'll sign for this PR and #2767 (and safe-global/safe-core-sdk#1439) once I've read it. Thanks!