Replace send_email from utils in task SDK with its own

[amoghrajesh]

closes: #54712

Why?

Task SDK uses the ability to send emails but it imports it from airflow.utils. As part of client server separation, getting rid of these imports in task sdk. This PR moves the _send_email function from utils to the task runner module to consolidate email-related functionality where it's actually used.

Changes

• Moved _send_email function from utils to task runner
• Added test to verify email functionality works correctly in the new location -- there was no task test that did this

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[ashb]

I wonder if we could replace this with the SMTP provider's SmtpNotifier in some way?

It seems odd to have two different ways of sending email.

We could fail at parse time if you set email_on_failure = True and the smtp provider can't be imported?

[amoghrajesh]

@ashb we could, but do we want to create a dependency between task sdk and smtp provider? I did this to avoid that

[ashb]

I think there is a qausi-dep there already -- SMTP provider was core, and you can define the credentials in an SMTP connection inside Airflow, can't you?

This doesn't need to be a hard (i.e. pyproject-level) dependency

[ashb]

I've thought about it before, and changing email_on_failure automatically to an SMTPNotifier seems like the way to go here. And if you set email_on_failure and the provider isn't importable, we can raise a nice error message to show up as a dag parse errror

[kaxil]

fyi: #47146 & https://lists.apache.org/thread/tbnf2rcrowrp3ktltrhrcjvc53k58oxx

[eladkal]

I am really not sure where are we going with the email/smtp integration.
My wish to get rid of it in Airflow 3 and treat it like any other notifier out there was not successful.
I am hoping we can get to a state where all email core related stuff are 100% deprecated and advise users migrate to the SMTP notifier. I don't think we need to continue build things in task sdk for emails.

[ashb]

Lets not port this over then.

Instead lets "upgrade in place" the use of email_on_failure attr etc and automatically convert that to an SmtpNotifier instance. And if the smtp provider isn't installed raise a nice error of "You need to install apache-airflow-providers-smtp to use this feature".

I'll leave @amoghrajesh do decide if this PR gets closed or re-purposed.

[amoghrajesh]

Thanks for all your comment, let's repurpose the existing SMTP notifier to do the email sending portion for us

[amoghrajesh]

Closing in favour of #57354