VPAAMP-454 Enable AAMP L1 AampGstPlayer tests

[jfagunde]

Reason for Change: Protect against regressions in AampGstPlayer code

Summary of Changes:

• Add AampGstPlayer L1 test directory
• Add L1 AampGstPlayerMain file
• Remove middleware code from L1 AampGstPlayer CMake file
• Delete tests that verified middleware code
• Update existing L1 tests
• Add middleware fakes

Test Procedure: Run AAMP L1 tests

Priority: P1

Risks: Low

[Copilot]

Pull request overview

Enables/repairs AAMP L1 coverage around aampgstplayer.cpp by reworking the AampGstPlayer test suite to build without pulling in the full middleware pipeline, and by introducing additional fakes to satisfy link-time dependencies.

Changes:

• Registers the AampGstPlayer L1 test target in the utests build and refactors its CMake to compile only aampgstplayer.cpp plus the local test sources.
• Removes middleware-focused test cases (including the default subtitle mute suite) and streamlines remaining AampGstPlayer L1 tests to use the shared_ptr-based mock globals.
• Adds new middleware fakes (notably FakeInterfacePlayerRDK) and a new AampGstPlayer gtest main().

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/utests/tests/CMakeLists.txt	Adds the AampGstPlayer subdirectory to the utests build.
test/utests/tests/AampGstPlayer/NullGuardTests.cpp	Updates fixture mock setup/teardown to use shared_ptr-based globals and removes middleware-only expectations.
test/utests/tests/AampGstPlayer/FunctionalTests.cpp	Removes middleware/pipeline-heavy tests and simplifies to basic AAMPGstPlayer behavior checks with updated mock plumbing.
test/utests/tests/AampGstPlayer/DefaultSubtitleMuteTests.cpp	Deletes middleware-implementation tests that no longer fit the intended L1 isolation boundary.
test/utests/tests/AampGstPlayer/CMakeLists.txt	Drops middleware sources/libs from the test build and adds a suite-local main().
test/utests/tests/AampGstPlayer/AampGstPlayerMain.cpp	Adds the gtest entry point for the AampGstPlayer test executable.
test/utests/fakes/FakeInterfacePlayerRDK.cpp	Introduces a stubbed InterfacePlayerRDK to allow aampgstplayer.cpp to link without real middleware.
test/utests/fakes/FakeCachedFragment.cpp	Adds a fake CachedFragment implementation (currently problematic due to collisions with existing CachedFragmentTests).