fix: relax memory_kv key validation to allow digits after first letter

[octo-patch]

Fixes #1272

Problem

The _is_valid_key_format method in memory_kv.py used a regex pattern ^[a-z]+(_[a-z0-9]+)*$ that required the first segment of a key to contain only lowercase letters. This incorrectly rejected valid snake_case keys that contain digits after the first character, such as:

• q1_report (quarterly reports)
• user123_profile (user profiles with IDs)
• v2_config (versioned configurations)
• s3_bucket_name (cloud resource names)

This caused false negative evaluation results during BFCL v4 multi-turn evaluation.

Solution

Updated the regex pattern from:

pattern = r"^[a-z]+(_[a-z0-9]+)*$"

to:

pattern = r"^[a-z][a-z0-9]*(_[a-z0-9]+)*$"

This change:

• Still requires the first character to be a lowercase letter (no digit-first keys)
• Allows digits after the first character in any segment
• Still rejects keys starting with uppercase, digits, spaces, or underscores
• Aligns with Python PEP 8 naming guidelines and real-world API naming patterns

Testing

Verified with manual tests:

• q1_report, user123_profile, v2_config, s3_bucket_name → now correctly accepted ✓
• MyKey, my key, _key, 1key, key_ → still correctly rejected ✓