Avoid Decimal objects in number_to_string

[bbayles]

This PR updates playlist serialization such that it avoids creating Python Decimal objects.

We want this input and output behavior:

25 -> '25'
25.0 -> '25'
25.1 -> '25.1'
0.000011 -> '0.000011'

We never want exponential notation, even for very small numbers. We do want integer values to look like integers, even if they come in as floats.

This PR:

• Formats the input number as a fixed point number with 6 places after the decimal
• Strips off trailing 0s
• Strips off trailing decimal points

The performance is 3x better this way. All existing tests pass, and our library of ~900 playlists from the wild show no change.

CC: @wurlced, @bendpham

[Copilot]

Pull request overview

This PR improves the performance of playlist serialization by replacing Decimal-based number formatting with a simpler string formatting approach that avoids creating Decimal objects.

Changes:

• Removed the decimal module import and refactored number_to_string() to use Python's built-in format() function
• Updated number_to_string() to accept a format specifier parameter for flexibility
• Simplified the StreamInfo.__str__() method to use the updated number_to_string() for frame rate formatting

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