Add Search service support with async/sync test coverage and docs updates

[jonathan343]

Overview

This PR adds Spotify Web API Search support to the SDK with full async/sync parity.

Summary

• Add AsyncSearchService / SearchService with:
  • search(q, types, market=None, limit=5, offset=0, include_external=None)
• Add typed SearchResult model with optional result pages for:
  • tracks, artists, albums, playlists, shows, episodes, audiobooks
• Wire client.search on async/sync clients.
• Add async/sync unit tests for validation, params, and typed parsing.
• Add async/sync integration tests for search using client credentials.
• Update docs and nav:
  • new docs/reference/services/search.md
  • clients/services/models references and README examples
• Add changelog entry for search service

Testing

Non-Integ Tests Passed

See GitHub CI for this PR.

Integ Tests Passed

(spotify-sdk) $ uv run pytest -m integration
================================= test session starts ==================================
platform darwin -- Python 3.14.2, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/gytndd/dev/personal/spotify-sdk
configfile: pyproject.toml
plugins: anyio-4.12.1, httpx-0.36.0
collected 453 items / 441 deselected / 12 selected                                     

tests/_async/integration/test_episode.py ..                                      [ 16%]
tests/_async/integration/test_library.py ..                                      [ 33%]
tests/_async/integration/test_search.py ..                                       [ 50%]
tests/_async/integration/test_show.py ..                                         [ 66%]
tests/_sync/integration/test_episode.py .                                        [ 75%]
tests/_sync/integration/test_library.py .                                        [ 83%]
tests/_sync/integration/test_search.py .                                         [ 91%]
tests/_sync/integration/test_show.py .                                           [100%]

========================= 12 passed, 441 deselected in 11.48s ==========================

References

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