tvOS: Set firmware version in User Agent string

[rakuco]

Pass the OS version as firmware version just like C25 did, otherwise this field would be set to "Unknown".

Bug: 442760345

[github-actions]

🤖 Gemini Suggested Commit Message

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tvos: Set firmware version in User Agent string

Set the firmware version for tvOS in the User Agent string.
Previously, this field was set to "Unknown". This change populates
it with the actual OS version, aligning with C25 behavior.

Bug: 442760345

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[rakuco]

cc @abhijeetk @jkim-julie @Gyuyoung

[gemini-code-assist]

Code Review

This pull request correctly sets the firmware version in the User Agent string for tvOS by using the operating system version. The change is simple, confined to the tvOS platform-specific initialization, and directly addresses the goal of preventing the firmware version from being reported as 'Unknown'. The implementation is straightforward and looks good.