fix: corrected FileLottie.hashCode to prevent cache memory leak

[BenjaminDucombs]

From issue #412:

Problem

FileLottie.operator== compared file.path, but hashCode returned
file.hashCode (the identity hash of the io.File object). This violated
the hashCode/== contract: two FileLottie instances pointing to the
same path were considered equal but produced different hash codes.

As a result, sharedLottieCache.putIfAbsent(this, ...) always missed on
a hash lookup, inserting a new cache entry every time the same file was
loaded. Over time this caused the cache to grow without bound, leaking
memory.

Fix

Change hashCode to use file.path.hashCode so it is consistent with
the equality comparison.

// before
int get hashCode => file.hashCode;

// after
int get hashCode => file.path.hashCode;

[xvrh]

Thanks for the contribution!