Add Object.to_dict() and CrossReference.__repr__ for JSON-esque output

[shaikn6]

Closes #881.

What

• _Object.to_dict() (inherited by Object, ObjectSingleReturn, GroupByObject) converts a query result into a fully JSON-serializable dict — handling datetime → isoformat, UUID → str, GeoCoordinate/PhoneNumber → JSON, and recursively expanding cross-referenced objects into their own to_dict() output.
• _CrossReference.__repr__ now shows its objects (CrossReference(objects=[...])) instead of the default <... object at 0x...> memory address.

Why

Per the issue and the existing comment on it, printing/serializing query results — especially ones with cross-references — currently either falls back to Python's default repr or requires hand-rolling a converter. to_dict() gives an explicit, opt-in way to get a JSON-serializable representation; the __repr__ fix makes ad-hoc print()/logging readable without calling anything.

Scope notes

• Kept to_dict() as an explicit method rather than overriding Object.__repr__/__str__, so existing scripts/logs that already print objects aren't affected — only CrossReference's repr changes.
• CollectionConfig already had its own to_dict() (added separately, for REST-payload serialization) — this PR doesn't touch that, it fills the gap on the query-result side (Object/CrossReference).
• No cycle-guard on the recursive reference expansion — Weaviate v4 query results aren't cyclic by default, so this wasn't added to avoid speculative complexity. Documented in the docstring.

Testing

• 4 new unit tests in test/collection/test_object_to_dict.py — all passing
• Full test/collection/ suite: 326 passed, 0 regressions
• Full test suite: 416 passed; 3 pre-existing unrelated failures in test/test_timeout.py reproduced identically on a clean checkout (not caused by this change)
• ruff check, ruff format --check, and flake8 (matching this repo's pre-commit config) all clean

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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