fix: Assume naive datetime as UTC in serialize_datetime

[Deng-m1]

## Summary

Fix timestamp inconsistency that causes duplicate trace records in ClickHouse when the SDK runs in non-UTC timezones.

Closes https://github.com/langfuse/langfuse/issues/11026

## Problem

When a datetime object without timezone info (naive datetime) is passed to `serialize_datetime()`, 
it was previously interpreted using the **local timezone**. In UTC+8 environments, this caused:

1. **Duplicate trace records** - Same trace ID gets different `toDate(timestamp)` values
2. **ReplacingMergeTree merge failures** - ClickHouse can't merge records with different dates
3. **Inconsistent trace rendering** - Traces appear fragmented in the UI

### Example

id: 2e0e8e5f811849f5106dc38a584cce27
Row 1: timestamp=2025-12-10 (UTC), name=NULL
Row 2: timestamp=2025-12-11 (local), name=LangGraph ← Different date!

## Root Cause

In `langfuse/api/core/datetime_utils.py`:

```python
# Before (problematic)
local_tz = dt.datetime.now().astimezone().tzinfo
localized_dt = v.replace(tzinfo=local_tz)

Solution

Assume naive datetime is UTC, consistent with:

• Langfuse infrastructure requirements (all timestamps should be UTC)
• Internal _get_timestamp() which already uses datetime.now(timezone.utc)

# After (fixed)
utc_dt = v.replace(tzinfo=dt.timezone.utc)

Testing

Added 11 test cases in tests/test_datetime_utils.py:

• ✅ UTC datetime serialization (ends with 'Z')
• ✅ Non-UTC timezone handling (uses offset format)
• ✅ Naive datetime assumed as UTC
• ✅ Consistency with _get_timestamp()
• ✅ Edge cases (midnight, end of day)
• ✅ ISO 8601 compliance

All tests pass.

Checklist

• Code follows project conventions
• Tests added for the fix
• All tests pass locally
• Linked to issue #11026

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> [!IMPORTANT]
> Fixes `serialize_datetime()` to assume naive datetimes as UTC, preventing duplicate trace records in ClickHouse.
> 
>   - **Behavior**:
>     - `serialize_datetime()` in `datetime_utils.py` now assumes naive datetimes are UTC, preventing duplicate trace records in ClickHouse.
>     - Aligns with Langfuse infrastructure requirements for UTC timestamps.
>   - **Testing**:
>     - Adds 11 test cases in `test_datetime_utils.py` to verify correct serialization of UTC, naive, and non-UTC datetimes.
>     - Tests include edge cases like midnight, end of day, and ISO 8601 compliance.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis" src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=langfuse%2Flangfuse-python&utm_source=github&utm_medium=referral)<sup> for 02587ef3a9ed726d4960df1698c387240761bc56. You can [customize](https://app.ellipsis.dev/langfuse/settings/summaries) this summary. It will automatically update as commits are pushed.</sup>

<!-- ELLIPSIS_HIDDEN -->

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**Disclaimer**: Experimental PR review
---

<h2>Greptile Overview</h2>

<h3>Greptile Summary</h3>


Fixed timestamp inconsistency causing duplicate trace records in ClickHouse when the SDK runs in non-UTC timezones by changing `serialize_datetime()` to assume naive datetimes are UTC instead of local time.

- Changed naive datetime handling in `langfuse/api/core/datetime_utils.py` from local timezone to UTC assumption
- This aligns with Langfuse infrastructure requirements where all timestamps should be UTC
- Consistent with internal `_get_timestamp()` which uses `datetime.now(timezone.utc)`
- Added 11 comprehensive test cases covering UTC, non-UTC timezones, naive datetime handling, and edge cases
- **Note:** This is a behavioral change - naive datetimes will now serialize differently in non-UTC environments

<h3>Confidence Score: 4/5</h3>


- Safe to merge - targeted bug fix with comprehensive test coverage.
- The change is well-documented, addresses a real issue (duplicate trace records), and includes thorough test coverage. Minor deduction for the in-method import style issue in tests.
- No files require special attention.

<h3>Important Files Changed</h3>



File Analysis



| Filename | Score | Overview |
|----------|-------|----------|
| langfuse/api/core/datetime_utils.py | 5/5 | Changed naive datetime handling from local timezone to UTC assumption, fixing duplicate trace records in ClickHouse for non-UTC environments. |
| tests/test_datetime_utils.py | 4/5 | Added 11 comprehensive test cases for datetime serialization; contains one in-method import that should be moved to module top. |

</details>



<h3>Sequence Diagram</h3>

```mermaid
sequenceDiagram
    participant SDK as Langfuse SDK
    participant DT as serialize_datetime()
    participant CH as ClickHouse

    Note over SDK,CH: Before Fix (Non-UTC timezone)
    SDK->>DT: naive datetime (e.g., 2025-12-10 21:00)
    DT->>DT: Apply local timezone (+08:00)
    DT->>CH: 2025-12-11T05:00:00+08:00
    Note over CH: Stored as 2025-12-11

    Note over SDK,CH: After Fix (Any timezone)
    SDK->>DT: naive datetime (e.g., 2025-12-10 21:00)
    DT->>DT: Assume UTC
    DT->>CH: 2025-12-10T21:00:00Z
    Note over CH: Stored as 2025-12-10 (consistent)

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