feat: add cross-encoder reranking to search

[Teagan42]

What

• add optional CrossEncoder reranker for search_media
• fetch extra candidates and reorder results by reranker scores
• cover reranking behavior with unit tests

Why

• improve search relevance while allowing reranker to be disabled for latency-sensitive scenarios

Affects

• mcp_plex/server.py
• tests/test_server.py

Testing

• uv run ruff check .
• uv run pytest

Documentation

• n/a

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https://chatgpt.com/codex/tasks/task_e_68b3ea23a1948328ad4b6849262dd88d

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mcp_plex
loader.py	179	10	94%	32–34, 296, 299–302, 523, 529
server.py	162	7	96%	298, 318–322, 326
TOTAL	443	17	96%

Tests	Skipped	Failures	Errors	Time
13	0 💤	0 ❌	0 🔥	6.614s ⏱️