[Platform][Cache] Key MessageBag inputs on their message content

[lazerg]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #2192
License	MIT

MessageBagCacheKeyGenerator returned $input->getId()->toString(), and MessageBag generates a fresh Uuid::v7() in its constructor. Every request builds a new bag, so every request produced a new cache key and CachePlatform never returned a cached result for a conversation.

The generator now hashes the messages themselves: their class, their content and their order. The identifiers and the metadata of the bag and of its messages stay out of the hash, since they describe the instance rather than the conversation. Identifiers a provider assigns to a content part, a tool call id for instance, are kept, because they are sent along with it. Binary content is keyed on a hash of its bytes, the way FileCacheKeyGenerator already does, so a lazily read file does not leak its closure into the key.

template_vars now goes into the key as well. Templates are rendered inside the inner platform, after CachePlatform has built the key, so without that a bag of templates would serve the first render to every later set of variables.

The existing test for two separate bags with the same messages asserted the current behaviour (two calls to the inner platform), so it now asserts the cache hit, and further tests cover two bags with different messages and two calls with different template variables.