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

rust/lancedb/src/function.rs

@@ -453,7 +453,8 @@ pub struct FunctionVersion {
     signature: FunctionSignature,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     secret_bindings: Vec<SecretBinding>,
-    created_at: String,
+    /// When this version was created, in milliseconds since the epoch.
+    created_at_millis: i64,
     metadata: BTreeMap<String, String>,
     disabled: bool,
 }
@@ -507,8 +508,9 @@ impl FunctionVersion {
         &self.secret_bindings
     }
 
-    pub fn created_at(&self) -> &str {
-        &self.created_at
+    /// When this version was created, in milliseconds since the epoch.
+    pub fn created_at_millis(&self) -> i64 {
+        self.created_at_millis
     }
 }
 

rust/lancedb/tests/fixtures/first_class_functions/v1/remote_function_job.json

@@ -23,7 +23,7 @@
         "nullable": false
       }
     },
-    "created_at": "2026-08-21T00:00:00Z",
+    "created_at_millis": 1787270400000,
     "image": {
       "manifest_digest": "sha256:7e22f815b6648e14f093a3979a8e5a2082fa773ebe1ec84b135cae7e84d6f8e6",
       "descriptor": {

rust/lancedb/tests/fixtures/first_class_functions/v1/remote_function_version.canonical.json

@@ -1 +1 @@
-{"created_at":"2026-08-21T00:00:00Z","disabled":false,"image":{"descriptor":{"behavior":{"result_stability":"input_and_context","side_effects":"non_idempotent"},"entrypoint":"app.function:create","format_version":1,"interface":{"type":"lance.scalar","version":1},"python":{"abi_tag":"cp312","executable":"/usr/local/bin/python3","implementation":"cpython","import_paths":["/opt/function/code"],"version":"3.12.14"},"python_api":1,"requires":{"capabilities":[],"kernel_min":"4.18.0"},"schemas":{"initialization":"/opt/function/schemas/initialization.arrow","input":"/opt/function/schemas/input.arrow","output":"/opt/function/schemas/output.arrow"}},"manifest_digest":"sha256:7e22f815b6648e14f093a3979a8e5a2082fa773ebe1ec84b135cae7e84d6f8e6","source":false},"location":"memory:///fixture","metadata":{},"name":"embed","object_id":"fixture","secret_bindings":[{"kind":"env","secret_ref":{"name":"hf-prod"},"variable":"HF_TOKEN"}],"signature":{"inputs":[{"arrow_type":"utf8","name":"text","nullable":true}],"output":{"arrow_type":"list<float32>","kind":"scalar","nullable":false}},"version":"1"}
+{"created_at_millis":1787270400000,"disabled":false,"image":{"descriptor":{"behavior":{"result_stability":"input_and_context","side_effects":"non_idempotent"},"entrypoint":"app.function:create","format_version":1,"interface":{"type":"lance.scalar","version":1},"python":{"abi_tag":"cp312","executable":"/usr/local/bin/python3","implementation":"cpython","import_paths":["/opt/function/code"],"version":"3.12.14"},"python_api":1,"requires":{"capabilities":[],"kernel_min":"4.18.0"},"schemas":{"initialization":"/opt/function/schemas/initialization.arrow","input":"/opt/function/schemas/input.arrow","output":"/opt/function/schemas/output.arrow"}},"manifest_digest":"sha256:7e22f815b6648e14f093a3979a8e5a2082fa773ebe1ec84b135cae7e84d6f8e6","source":false},"location":"memory:///fixture","metadata":{},"name":"embed","object_id":"fixture","secret_bindings":[{"kind":"env","secret_ref":{"name":"hf-prod"},"variable":"HF_TOKEN"}],"signature":{"inputs":[{"arrow_type":"utf8","name":"text","nullable":true}],"output":{"arrow_type":"list<float32>","kind":"scalar","nullable":false}},"version":"1"}