[FEATURE] Assembly AI - Improve actions & sources

[vunguyenhung]

Is your feature request related to a problem? Please describe.

1. Descriptions of AssemblyAI actions are incorrect and confusing

• Get transcription: "Fetches a specific transcribed result from the AssemblyAI API."
• Actually should be named as "Transcribe audio" and description is "Create a transcript from a media file that is accessible via a URL."
• Related API Doc: https://www.assemblyai.com/docs/api-reference/transcripts/submit

2. Add action "Get transcription" that actually will execute "GET" of transcription like in docs:
   https://www.assemblyai.com/docs/api-reference/transcripts/get

3. Trigger: New Transcription Completed
   If possible include transcription text in trigger data

Additional context
Raised by a user here: https://pipedream-users.slack.com/archives/CMZG4EBJ9/p1753863253454019

[vunguyenhung]

Hi everyone, all test cases are passed! Ready for release!

Test report
https://vunguyenhung.notion.site/FEATURE-Assembly-AI-Improve-actions-sources-240bf548bb5e81adbbabfb2ae4d1eadf