fix: make MemoryConfig.field_weights optional

[mjnovice]

Summary

• Make field_weights optional (dict[str, float] | None, default None) instead of required with a strict validator
• Remove the _validate_field_weights model validator that raised ValueError for empty dicts

Why

When no fieldSettings are configured in the agent definition's dynamicFewShotSettings, the runtime passes field_weights={} which fails validation. In practice, memory spaces may not have field weights configured — in that case, all input fields should be searched with default weights.

This was causing AGENT_STARTUP.UNEXPECTED_ERROR in production:

1 validation error for MemoryConfig
  Value error, field_weights must contain at least one field

Test plan

• Agents with no fieldSettings configured can start without error
• Agents with fieldSettings still work as before (weights are passed through)

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