Rename Association#object_id to #xero_object_id to avoid shadowing Kernel#object_id

[peterc]

Summary

• Renames object_id to xero_object_id on the XeroRuby::Files::Association model to avoid shadowing Kernel#object_id, which is a core Ruby method that should never be overridden.
• The JSON serialization key (ObjectId) is unchanged, so wire compatibility is preserved.
• API method signatures in FilesApi are unaffected (they use object_id as a local parameter name, not a model accessor).

Why this matters

Defining attr_accessor :object_id on any Ruby object overrides Kernel#object_id, which Ruby uses internally for object identity. This causes a warning (redefining 'object_id' may cause serious problems) and can lead to subtle bugs in equality checks, hash lookups, and debugging tools.

[peterc]

Fixes #327 and #348 BTW

[rept]

Hope this can get merged soon.