fix: improve ISO datetime parsing

[zztdandan]

Description

Improve ISO 8601 datetime parsing to accept Z-suffix timestamps and trim excessive fractional seconds, then use this parser across core read paths.

Related Issue

Fixes #

Root Cause

Two related parsing gaps caused valid timestamps to fail:

• datetime.fromisoformat() does not accept Z suffix (e.g., 2026-03-03T01:26:14.481Z).
• Windows-style timestamps can include more than 6 fractional digits, which fromisoformat() rejects.

These inputs appear in stored context/message timestamps, leading to ValueError during deserialization.

Changes Made

File	Change
openviking/utils/time_utils.py	Normalize Z -> +00:00 and trim excessive fractional seconds before parsing.
openviking/core/context.py	Use parse_iso_datetime() for created_at and updated_at.
openviking/message/message.py	Use parse_iso_datetime() for message created_at.
openviking/retrieve/hierarchical_retriever.py	Use parse_iso_datetime() when parsing updated_at.
tests/unit/test_time_utils.py	Added unit tests for Z-suffix parsing and context deserialization.

Design Decisions

• Reuse existing utility: parse_iso_datetime() already handled excess fractional seconds; the fix extends it to accept Z.
• Backward compatible: Inputs already accepted by fromisoformat() still parse correctly.
• Minimal surface area: Only the affected parsing call sites were updated.

Testing

Before:

• Context.from_dict({"created_at": "2026-03-03T01:26:14.481Z"}) -> ValueError

After:

• parse_iso_datetime("2026-03-03T01:26:14.481Z") -> timezone-aware datetime
• Context.from_dict(...) -> parses without error

Tests:

• python -m pytest tests/unit/test_time_utils.py

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Checklist

• My code follows the project coding style
• I have performed a self-review of my code
• New and existing unit tests pass locally with my changes
• My changes generate no new warnings

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