Visibility controls on public API

[alan-george-lk]

This PR changes the following:

• Define LIVEKIT_API macro to expose LiveKit API symbols across platforms (common convention, example)
• All symbol visibility hidden by default otherwise
• Update relevant class/method symbols with this new macro
• Made include prefixes consistent for public API (all now use #include "livekit/<blah" convention, updated AGENTS.md to define this rule
• Hide third-party symbols from consumers
  • spdlog was installed at the system level in CI (and documented for users to install at that level), but was brought in via FetchContent/vcpkg. This is no longer needed (system install). Removed that flow and deprecated the CMake argument/functionality for using system level spdlog. It is now entirely within the SDK and fully private/no symbols exposed to clash

Testing

• Built against/linked in downstream project that originally exposed this issue (suspected spdlog clash with third-party lib)
  • Without this fix and conflicting spdlog: process has deied [pid 5915, exit code -7...
• Added tester script to tests.yml CI stage to catch this across all OS moving forward, with the small caveat that we need to ensure any new deps are added to the symbol check
• Unit/integration tests are standalone binaries that practice this linking, ensured they still worked via CI

[stephen-derosa]

LGTM -- is there any test we can run to make sure we arent missing the LIVEKIT_API macro for items that should be public?

[xianshijing-lk]

lgtm assuming the check will be moved to release CI flow