refactor(core): centralize headers for streamable file responses

[ankitbelal]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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

What kind of change does this PR introduce?

• 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?

Currently, StreamableFile responses have headers set directly in multiple places in the ExpressAdapter, which leads to duplicate logic and ill maintained and lengthy method.

Issue Number: N/A

What is the new behavior?

Headers for StreamableFile responses are now centralized into private helper methods (setHeaderIfNotExists and applyStreamHeaders).
-Makes it easier to maintain and extend header logic in the future
-Reduce duplicate code

Does this PR introduce a breaking change?

• Yes
• No

Other information

This is purely a refactor. No new features or bug fixes included.

[coveralls]

Pull Request Test Coverage Report for Build 6aed0320-689c-485f-812a-537dde8a983e

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 89.841%

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Totals
Change from base Build 982ac74a-bb1f-4e45-9daf-0a5fe420033d:	0.0%
Covered Lines:	7473
Relevant Lines:	8318

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💛 - Coveralls