Fix missing user_instructions in LongFormContentStrategy prompt params

[mrickar]

Description:
Previously, LongFormContentStrategy’s prompt_param_composer did not include user_instructions from config_conversation.
As a result, in podcastfy/content_generator.py, the following line:

prompt_params["user_instructions"] = prompt_params.get("user_instructions", "") + self.LONGFORM_INSTRUCTIONS

would overwrite the user_instructions input instead of appending to it.

This PR updates the composer to correctly include user_instructions from config_conversation, ensuring that the final prompt preserves user-provided instructions along with LONGFORM_INSTRUCTIONS.

Impact:

• Fixes overwritten user instructions in long-form content generation
• Ensures consistent behavior across strategies