Add timeout/retry to client config fetch; don't cache failures

[sujen1412]

Fixes #135

Changes

• _get_client_config now uses a requests.Session with urllib3.Retry (exponential backoff on connection errors and 502/503/504) and a (5, 30)s connect/read timeout.
• Replaced @cache with a dict that stores only successful results. Failures raise a RuntimeError naming the config URL instead of returning None (which previously caused a cached failure and a later AttributeError).
• Added tests: transient failure followed by success, permanent failure not cached, and success cached with timeout passed.

Notes

• Existing tests in test_config_reader.py leak MAAP_API_HOST_SCHEME into the environment, which makes some URL tests order-dependent; the new tests use explicit https:// hosts to avoid that. I did not touch those.
• Only the new tests plus URL tests were run locally (no network access to the hosted-API tests).

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https://claude.ai/code/session_01DZUGQ3FvoiU3EkUrvAPTTQ

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