fix(service): allow fastly service version validate to use FASTLY_SERVICE_ID env fallback

[Tyagiquamar]

Change summary

Fixes #1866

Allow \astly service version validate\ to fall back to the \FASTLY_SERVICE_ID\ environment variable when the --service-id\ flag is omitted or when running in non-interactive/automation environments.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
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User Impact

Users executing \astly service version validate\ can rely on \FASTLY_SERVICE_ID\ without needing to pass --service-id\ explicitly on each execution.

Are there any considerations that need to be addressed for release?

None.

[jedisct1]

Thanks for your contribution.

Cab you add a ### Bug Fixes entry under [Unreleased], for example:

- fix(service): allow `service version validate` to use `FASTLY_SERVICE_ID`.

?

Also, maybe renamed “missing service ID” test should explicitly set:

EnvVars: map[string]string{"FASTLY_SERVICE_ID": ""},

Thank you!

[jedisct1]

Thanks!

[Tyagiquamar]

Thanks! I’ll make the requested changes and push an update shortly.

[rcaril]

Hi @Tyagiquamar - thanks for your contribution! Please use our PR template for the initial description.

### Change summary

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Briefly describe the changes introduced in this pull request. Include context or
reasoning behind the changes, even if they seem minor. If relevant, link to any
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All Submissions:

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* [ ] Have you checked to ensure there aren't other open [Pull Requests](https://github.com/fastly/cli/pulls) for the same update/change?

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### New Feature Submissions:

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### Changes to Core Features:

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### User Impact

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### Are there any considerations that need to be addressed for release?

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[Tyagiquamar]

Updated the PR description to match the template. Both the changelog entry and the explicit empty env test have also been added.

[Tyagiquamar]

Hi @jedisct1, friendly ping when you have a moment. The PR template, changelog entry, and explicit empty env test have all been added, and CI checks are passing. Thanks!

[jedisct1]

LGTM, thank you!