feat(cache): add opt-in sort_query to merge reordered query strings

[allen0099]

Closes #267

What

@cache(sort_query=True) makes ?limit=1&q=wid and ?q=wid&limit=1 share one cache entry. The default False leaves every existing key byte-for-byte unchanged.

How the query is sorted

• The key's query component was already str(request.query_params): Starlette parses the query (blank values kept, empty && segments dropped, names and values percent-decoded) and re-encodes the pairs in the order sent. sort_query stable-sorts those same decoded (name, value) pairs by name and encodes them with the same urlencode, so only the order changes. A query that is already in order gets the identical key with or without the flag.
• Sorting the raw segments instead would have encoded differently from the unsorted key (for example %20 against +), and would split requests that the default key already treats as equal.
• Names are compared decoded, because the key already writes %61 as a. Sorting by the raw name would put %61=1 apart from the other a values even though the key shows them as the same name.
• The sort is stable (key=itemgetter(0)), so ?tag=b&tag=a stays distinct from ?tag=a&tag=b: a handler reading tag: list[str] sees a different order.

Validation and other entry points

• sort_query must be a bool. Combining it with a custom key_builder raises CacheXError when the decorator is applied (the same style as the other decoration-time checks), and the message points to build_cache_key(request, sort_query=True). build_cache_key gains that keyword-only parameter, so a custom builder can opt in itself.
• invalidate() gains a keyword-only sort_query, validated the same way. Like key_builder and vary, it cannot read the route's settings, so the caller passes the route's value. This is documented, and a test pins that omitting it misses the sorted entry.
• clear_path() matches on the path component only, so it is unaffected. A test covers this.

Docs

• docs/HTTP_CACHING.md: the "query order matters" note now describes sort_query, and the invalidate() section is updated.
• The zh-TW translation is updated to match.
• The @cache, build_cache_key and invalidate docstrings are updated.
• Changelog fragment changelog.d/267.added.md.

Tests

tests/test_cache_sort_query.py covers:

• reordered parameters sharing one entry (sync and async handlers);
• repeated-name order staying distinct;
• the default keeping reordered queries apart;
• only order being merged;
• edge cases: empty query, &, &&, a segment without =, blank values;
• percent-encoded names and values kept as they were;
• decoded-name comparison;
• the custom key_builder combination and non-bool values being rejected;
• invalidate() with a reordered query;
• clear_path().

Mutation checks:

• Sorting by name and value (an unstable order for repeated names) fails 4 tests.
• Ignoring the flag in the key fails 15 tests.
• Always choosing the default builder fails 6 tests.