feat!: carry a FunctionVersion's creation time as epoch millis (#4482)

Moves Function timestamps on the wire from RFC 3339 strings to epoch
milliseconds.

## What

- `FunctionVersion.created_at` (an RFC 3339 string) becomes
`created_at_millis` (milliseconds since the epoch, as an integer).
That's the shape Secrets, jobs and `FunctionErrorRecord` already use.
- Rust: `FunctionVersion::created_at() -> &str` becomes
`created_at_millis() -> i64`.
- Python: `FunctionVersion.created_at: str` becomes `created_at_millis:
int`.
- Fixtures and the mock server now send `1787270400000`, the old
`2026-08-21T00:00:00Z`.

Node.js doesn't expose `FunctionVersion`, so it doesn't change.

## Compatibility

Breaking, and it must ship together with the matching server change: a
server that still sends `created_at` fails to deserialize on this
client. Functions aren't deployed yet, so nothing is migrated.

## Tests

- `cargo test --features remote --test first_class_function_slice1`: 10
passed
- `cargo test --features remote --lib function`: 51 passed
- `pytest python/tests/test_first_class_function*.py`: 100 passed
- clippy, `cargo fmt --check`, ruff check and ruff format are clean

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01BY5xcEGEAa1E11VG55Kd1w

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

python/python/lancedb/functions.py

@@ -372,7 +372,7 @@ class FunctionVersion(_RemoteValue):
     image: FunctionImage
     signature: FunctionSignature
     secret_bindings: tuple[SecretBinding, ...] = ()
-    created_at: str
+    created_at_millis: int
     metadata: Mapping[str, str]
     disabled: bool
 

python/python/tests/test_first_class_function_slice2.py

@@ -1555,7 +1555,7 @@ def _mock_remote_function_catalog():
                     )["image"],
                     "signature": body["signature"],
                     "secret_bindings": body.get("secret_bindings", []),
-                    "created_at": "2026-08-21T00:00:00Z",
+                    "created_at_millis": 1787270400000,  # 2026-08-21T00:00:00Z
                 }
                 response = {"job_id": "job-register"}
                 status = 202