feat: parse flag values strictly in built-in providers

[aubes]

Built-in providers no longer cast flag values silently:

• EnvVarProvider and RedisProvider parse raw values strictly. An invalid value (e.g. "banana" as boolean, "10.0" as integer) now returns the default value with PARSE_ERROR, instead of being cast.
• InMemoryProvider returns TYPE_MISMATCH when the YAML type does not match the requested type. The only accepted conversions are 0/1 as boolean and integers as float, like typed providers such as flagd.
• ResolutionDetailsTrait::toBool() is replaced by parseBool(), parseInt(), parseFloat() and parseObject().
• RedisProvider now logs client failures at error level. Previously, a Redis outage silently resolved every flag to its default value.

The docs list the accepted values per type for each provider.