Add audiobook search parity

[guidefari]

Parent PRD

#9

What to build

Extend search parity to support audiobook results once the audiobook domain types and schemas exist. This keeps search aligned with newer Spotify result types without coupling the work to the full audiobooks module implementation.

Acceptance criteria

• Search supports the audiobook type in request options and response decoding
• Runtime schemas decode audiobook search payloads returned by Spotify
• Tests cover observable audiobook search behavior

Blocked by

• Blocked by Improve public API documentation and examples #34

User stories addressed

• User story 1
• User story 2
• User story 16
• User story 17
• User story 37

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Migrated from guidefari/spotify-effect#56