Fix Alibaba OSS provider: configurable endpoint and task handler log …

[hasangzc]

Summary

• OSSHook._get_client(): The endpoint was hardcoded to the public URL
  (oss-{region}.aliyuncs.com), making it impossible to use VPC/internal
  endpoints (e.g. oss-eu-central-1-internal.aliyuncs.com). Now reads from
  the connection's extra config with endpoint key, falling back to the
  public default.

• OSSTaskHandler._read(): Called self.oss_log_exists() and self.oss_read()
  which don't exist on OSSTaskHandler. These methods live on OSSRemoteLogIO
  and must be accessed via self.io. This caused the webserver to fail silently
  when reading logs from OSS.

Test plan

• Added test_get_client_uses_default_endpoint - verifies default endpoint behavior
• Added test_get_client_uses_custom_endpoint_from_connection - verifies custom endpoint from extra config
• Added test_read_calls_oss_methods_via_io - verifies _read() uses self.io
• Added test_read_falls_back_to_local_when_no_remote_log - verifies local fallback

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Was generative AI tooling used to co-author this PR?

• Yes

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