[Regression] Incorrect optional map parsing of metadata streams?

[Mive]

Summary of the bug:
When specifying a stream with metadata stream specifiers that include a ? at the end, an error is raised Stream map '' matches no streams. No error is raised when removing the ?. This issue does not present itself when using normal stream speficiers like -map 0:s?. This issue is also not present in ffmpeg 7.0

More specifically, function stream_specifier_parse only specifies : as the value termniator instead of :? or similar, and thus the resulting key:value pair looks like this key:value? when a ? is present.

How to reproduce:
Get a video file with tracks containing language metadata and try to use a metadata stream specifier with a ? eg. -map 0:s:m:language:eng?

% ffmpeg -i input.mkv -map 0:V -map 0:s:m:language:eng? ... output.mkv

[Mive]

Full log output of attempted encode.

[MasterQuestionable]

͏    Spec for reference:
͏    https://ffmpeg.org/ffmpeg.html#Advanced-options

͏    "stream_specifier_parse" ":"
͏    ​https://github.com/FFmpeg/FFmpeg/blob/efa0c6252faa66ab90d0d5e60b0d4305474b7a8e/fftools/cmdutils.c#L1160

͏    The metadata name/value could contain literal "?", as well?
͏    ​https://trac.ffmpeg.org/ticket/11248#comment:2
͏    ​https://github.com/FFmpeg/FFmpeg/commit/9e68b26765ca8396050f4b58d4b4fdc53943d35f

[Mive]

I see, when I set it as -map 0:s:m:language:eng:?, it works as intended. In that case, the "optional map" example at https://ffmpeg.org/ffmpeg.html#Advanced-options should probably be updated to reflect the updated syntax, or add an additional example "optional map by language", as the -map 0:a? syntax still seems to work.