fix(microservices): support custom strategy in async usefactory config

[mag123c]

PR Checklist

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

When using dynamic microservice configuration with useFactory, any strategy field returned from the factory function is ignored.
This makes it impossible to provide a custom transport strategy dynamically.

Issue Number: #15144

What is the new behavior?

NestMicroservice now properly checks for a strategy field after resolving AsyncMicroserviceOptions.useFactory().
Adds test cases to ensure both static and dynamic strategy injection paths work as expected.

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build cce34446-2ec7-40ad-b756-d2995a48a213

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.94%

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Totals
Change from base Build 16a83cd7-0397-4555-852d-e8667ea8a43e:	0.0%
Covered Lines:	7197
Relevant Lines:	8092

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💛 - Coveralls

[kamilmysliwiec]

lgtm