WIP - [Perf] Migrate unswizzle-based unit tests to SPM (#11903)

[JesusRojass]

Addresses #11903

This migrates the unswizzle-based FirebasePerformance unit tests to run under Swift Package Manager. These suites currently run only through the CocoaPods pod gen based CI job, so their coverage disappears once CocoaPods tooling is retired.

Discussion

The seven gated suites (selector/class instrumentor, instrument, instrumentation, NSURLSession, NSURLConnection, and UIViewController instrumentation tests) were excluded from SPM with #if !SWIFT_PACKAGE when SPM support landed in #8412. The underlying reason is that unswizzling in FPRSelectorInstrumentor was compiled only when UNSWIZZLE_AVAILABLE was defined, which only the CocoaPods path wired up via the FPR_UNSWIZZLE_AVAILABLE env var and a test dependency on GoogleUtilities/SwizzlerTestHelpers.

Changes:

• FPRSelectorInstrumentor now detects the GULSwizzler unswizzle category at runtime instead of using the compile-time flag. Behavior for production apps is unchanged: the category is absent, so unswizzle still hits the same assertion as before.
• The PerformanceUnit SPM test target links GULSwizzlerTestHelpers (and GULMethodSwizzler), making the category available to tests.
• All #if !SWIFT_PACKAGE test gates are removed, plus the #ifdef UNSWIZZLE_AVAILABLE gates in FPRCounterListTest.
• FPRUIViewControllerInstrumentTest no longer depends on a host app key window; the shared application and key window are stubbed so the test is deterministic both hosted (CocoaPods) and unhosted (SPM).
• Removes the now-unused FPR_UNSWIZZLE_AVAILABLE wiring from the podspec, scripts/build.sh, and generate_project.sh.

Testing

• SPM: PerformanceUnit scheme on iOS and tvOS simulators
• CocoaPods: scripts/build.sh Performance-iOS-unit (no longer needs the env var)
• Watch items: ObjC category linking from the static helpers library under SPM, and the NSURLSession suite running unhosted (hermetic server plus run loops)

API Changes

• No API changes

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