Adapt for SMTP without auth (GSI-949)

[TheByronHimes]

This PR enables running the service with empty auth credentials. Testing was slightly improved by moving the SMTP instantiation to a separate method that can then be mocked.

[coveralls]

Pull Request Test Coverage Report for Build 10354400973

Details

• 27 of 27 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.7%) to 85.214%

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Totals
Change from base Build 10348389921:	1.7%
Covered Lines:	219
Relevant Lines:	257

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💛 - Coveralls

[lkuchenb]

Looks good to me. I think you can get around your dual-None validation and checking by modelling the config accordingly:

class SmtpAuthConfig(BaseModel):
    username: str
    password: str

class SmtpClientConfig(BaseSettings):
    smtp_host: str
    smtp_port: int
    smtp_auth: None | SmtpAuthConfig

You would then need to set env_nested_delimiter = '__' or alike to enable configuring the nested fields through environment variables. That way you can skip the validator and a conditional check whether auth is required would also be more straight forward!