osn-replay-buffer: invoke output handler to save replays

[sandboxcoder]

Description

• makes test more reliable resulting in much less replay buffer failures on the slower CI machines
• follows similar pattern used by OBS - ReplayBufferSave
• enable this test for MacOS now that it is more reliable

The previous approach can silently do nothing if:

• The hotkey type check (OBS_HOTKEY_REGISTERER_OUTPUT) doesn't match
• if no routed callback is registered, the save is a no-op
• The hotkey hasn't been registered yet (possible race condition on slow CI machines)

Motivation and Context

Improve consistency of replay buffer save unit test

How Has This Been Tested?

Ran it on build machine a lot of times and did not observe this test fail. Also tested in Desktop and verified replays save properly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Copilot]

Pull request overview

This PR updates the replay-buffer save implementation to invoke the underlying OBS output proc handler directly (rather than routing through a hotkey callback), with the goal of making replay saves—and the corresponding unit test—more reliable on slower CI machines. It also enables the advanced replay-buffer tests on macOS by removing the Darwin skip.

Changes:

• Reworked IReplayBuffer::Save to locate the replay buffer output and call its proc handler "save".
• Removed macOS skips for the two advanced replay-buffer start/save test cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/osn-tests/src/test_osn_advanced_replayBuffer.ts	Enables advanced replay-buffer tests on macOS by removing Darwin skips.
obs-studio-server/source/osn-replay-buffer.cpp	Implements replay save via the OBS output proc handler ("save") on the replay buffer output instance.

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