fix(cache-manager): return the round-tripped value from get_or_set on a miss

[allen0099]

Summary

CacheManager.get_or_set() returned the factory's object on a miss and the JSON-decoded copy on a hit:

miss: (1, 2)      hit: [1, 2]
miss: {1: 'a'}    hit: {'1': 'a'}

A miss now encodes the value once with the same _encode() that set() uses, stores those bytes, and returns json.loads of the same bytes. It does not serialise twice, and both paths give the same result.

• The contract is otherwise unchanged. A backend set error still propagates, as it did through self.set(). There is no fail-open path in CacheManager.
• A non-JSON-serialisable value (datetime, Decimal, UUID, a pydantic model) still raises TypeError after the factory has run, and nothing is stored. This can't be avoided, so it is now documented.
• add() returns a bool, not the value, so it doesn't have this inconsistency. No other method returns the stored value.
• Docs: a new "JSON round-trip" section in docs/APP_CACHE.md and its zh-TW mirror, with a table of how each type reads back.
• Typed reads (CacheManager: typed reads, incr, and get_many/set_many #249) are out of scope.

Tests

• tuple, int-keyed dict and nested values: the miss result equals the hit result, including its type. These run on memory and live Redis.
• the value is encoded exactly once, and the stored bytes are that encoding.
• a non-serialisable value raises TypeError, the factory runs once, and the key stays free.

Including the live Redis and Memcached tests: 1195 passed, 1 skipped, total coverage 99%, manager.py 100%.

Changelog

changelog.d/235.fixed.md

Closes #235