fix(audio): Normalize 'x-wav' audio format to 'wav'

[akshatvishu]

Closes #8996

Description:

The dspy.Audio.from_file (and from_url) method relies on Python's mimetypes.guess_type() to determine the audio format. On some operating systems, this function can return non-standard MIME types, such as audio/x-wav for .wav files.

These non-standard format strings, often prefixed with x- (like x-wav or x-m4a), are then passed to the LLM API (e.g., OpenAI). This can cause a 400 BadRequestError, as the API typically only accepts compliant formats (e.g., wav, m4a).

This patch adds a check to from_file, from_url, and the data URI branch of encode_audio to normalize these formats by removing any x- prefix, ensuring an API-compliant format is always sent.

[TomeHirata]

Thanks @akshatvishu, can you add a unit test?

[akshatvishu]

@TomeHirata Added the unit-test and also I slightly changed the logic and used the removeprefix() instead of the replace() to safely remove only the prefix from audio format strings, preventing unintended replacements if "x-" appears elsewhere in the format.