fix(player): respect stream user agent headers

[WhiteGiso]

Summary

Respects stream-provided User-Agent headers during Android playback.

The Android player now only applies the default player user agent when the stream does not already provide its own User-Agent header.

PR type

• Reproducible bug fix
• UI glitch/bug fix
• Behavior bug/regression fix
• Small maintenance only, with no UI or behavior change
• Docs accuracy fix
• Translation/localization only
• Approved larger or directional change

Why

Some streams require a specific User-Agent header to be accepted by the server.

The Android player previously set its default user agent even when the stream already provided a User-Agent header. This could cause playback requests to use the wrong user agent and fail.

This fix preserves stream-provided request headers by only applying the default user agent when no User-Agent is present.

Issue or approval

Fixes #1246

UI / behavior impact

• No UI change
• No behavior change
• UI changed only to fix a documented glitch/bug
• Behavior changed only to fix a documented bug/regression
• UI change has explicit maintainer approval
• Behavior change has explicit maintainer approval

Policy check

• I have read and understood CONTRIBUTING.md.
• This PR is small, focused, and limited to one problem.
• This PR is not cosmetic-only.
• Any UI change fixes a linked glitch/bug and includes visual proof, or this PR has no UI change.
• Any behavior change fixes a linked bug/regression or has explicit approval, or this PR has no behavior change.
• This PR does not bundle unrelated refactors, cleanups, formatting, or drive-by changes.
• This PR does not add dependencies, architecture changes, migrations, or product-direction changes without explicit approval.
• I listed the testing performed below.

Scope boundaries

This only changes Android playback header handling for streams that already provide a User-Agent.

It does not add new headers, change UI, change stream selection, add settings, add dependencies, or alter request headers other than avoiding the default user agent override when a stream-specific User-Agent exists.

Testing

• Ran git diff --check.
• Reviewed the Android playback data source setup to confirm the default user agent is still applied when no stream User-Agent is provided.
• Verified the change is limited to PlayerPlaybackNetworking.kt.

Manual validation needed on an Android device/build:

• Play a stream that provides a required custom User-Agent header.
• Play a normal stream without a custom User-Agent to confirm the default user agent is still used.
• Confirm playback requests are not rejected by hosts that require the stream-provided user agent.

Screenshots / Video

Not a UI change.

Breaking changes

None.

Linked issues

Fixes #1246