fix: deep-copy cache entries so mutating search results cannot corrupt the cache

[gaoflow]

Problem

Table.search stored and returned shallow copies of the document
list (docs[:]). The Document objects inside the cache and the
list returned to the caller were therefore the same objects.
Mutating a nested mutable value (e.g. appending to a list field) in a
returned document silently corrupted the cached results, causing
subsequent identical searches to return wrong data.

Minimal reproduction:

from tinydb import TinyDB, Query
from tinydb.storages import MemoryStorage

db = TinyDB(storage=MemoryStorage)
db.insert({'name': 'Alice', 'tags': ['a', 'b']})

Q = Query()
results = db.search(Q.name == 'Alice')
results[0]['tags'].append('c')          # mutate the returned document

# Second search hits the cache — should still return ['a', 'b']
print(db.search(Q.name == 'Alice'))     # prints [{'name': 'Alice', 'tags': ['a', 'b', 'c']}]

This was reported in issue #516.

Fix

Use copy.deepcopy when writing to and reading from
_query_cache, so cached entries and returned results are fully
independent. The change is two lines in Table.search:

# store
self._query_cache[cond] = deepcopy(docs)

# retrieve
return deepcopy(cached_results)

Tests

A new test test_query_cache_not_corrupted_by_mutation is added
alongside the existing test_query_cache. All 217 tests pass.