Add support for entry variants branch

[reeshika-h]

Introduce support for entry variants and branch handling in API requests. Update documentation with new guidelines for development workflow and coding standards. Remove outdated files and consolidate relevant information into new documentation files. This addresses improvements outlined in issue DX-6046.

[Copilot]

Pull request overview

This PR adds first-class support for requesting entry variants (via x-cs-variant-uid) and optionally scoping those requests to a branch (via branch) across Entry and Query by moving the API onto the shared EntryQueryable surface. It also expands the test suite with unit coverage for header construction and introduces a new API test + DVR cassette for variants.

Changes:

• Adds variants(uid:), variants(uids:), and branch-aware overloads on EntryQueryable to set the appropriate request headers.
• Updates unit tests for Entry and Query to validate variant/branch header formation.
• Adds EntryVariantsAPITest plus a DVR cassette intended for replay/recording.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Sources/EntryQuaryable.swift	Adds the new variants + branch header APIs on EntryQueryable.
Sources/Entry.swift	Removes now-redundant variants(...) methods from Entry (covered via EntryQueryable extension).
Tests/QueryTest.swift	Adds unit tests for Query.variants(...) header behavior and updates allTests.
Tests/EntryTest.swift	Adds unit tests for Entry.variants(..., branch:) header behavior.
Tests/EntryVariantsAPITest.swift	Adds integration-style tests for variants + branch, with DVR/live modes.
Tests/DVRRecordings/EntryVariants.json	Adds a DVR cassette for variants API calls.
ContentstackSwift.xcodeproj/project.pbxproj	Registers the new API test file in the Xcode project/test targets.

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