feat: add SPM traits for 3rd-party integrations

[github-actions]

Note

The pull request "feat: add SPM traits for 3rd-party integrations" was created by @itaybre but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Summary

• Add PrecompiledSentry (default) and SentryFromSource mutually exclusive package traits to all 4 3rd-party integrations (SentryCocoaLumberjack, SentryPulse, SentrySwiftLog, SentrySwiftyBeaver)
• Users can now select between precompiled Sentry xcframeworks (PrecompiledSentry, the default) and building from source (SentryFromSource) via swift test --traits SentryFromSource or by enabling the trait in their dependency declaration
• CI test matrix expanded to test both traits

Changes per integration

• Package.swift: bumped to swift-tools-version:6.1, added traits, conditional product dependencies (Sentry vs SentrySPM), and swiftSettings with trait-based defines
• Sources/*.swift: conditional import SentrySwift (from-source) vs import Sentry (precompiled)
• Tests/*.swift: same conditional import
• Sources/SentryTraitValidation.swift: #error if both traits enabled simultaneously

CI

• Added trait matrix dimension (PrecompiledSentry, SentryFromSource) to existing test-integrations-spm job
• Precompiled-only steps (download xcframeworks, prepare-package.sh) are skipped for SentryFromSource entries

Test plan

• All 4 integrations pass swift test (default PrecompiledSentry trait)
• All 4 integrations pass swift test --traits SentryFromSource
• CI passes with the new matrix

#skip-changelog

[linear-code]

COCOA-1825