fix: handle empty list for accounts field in configuration

[ZdenekSrotyr]

Summary

Fixes a Pydantic validation error when the UI sends an empty list [] for the accounts field instead of an empty dict {}. This was observed when loading Instagram accounts with no accounts configured.

The error was:

pydantic_core._pydantic_core.ValidationError: 1 validation error for Configuration
accounts
  Input should be a valid dictionary [type=dict_type, input_value=[], input_type=list]

Added a field_validator with mode="before" that normalizes the input:

• None → {}
• [] (empty list) → {}
• Non-empty list → raises TypeError (not supported)
• Dict → passed through unchanged

Review & Testing Checklist for Human

• Test Instagram V2 with no accounts configured - verify the component no longer crashes with the validation error
• Test Facebook Ads V2 and Facebook Pages V2 - ensure existing configurations with accounts as dict still work correctly
• Check if any existing configs send accounts as a non-empty list - the validator will now reject these with a TypeError; if such configs exist, we may need to handle list-to-dict conversion

Notes

This Configuration class is shared by all three components (Facebook Ads V2, Facebook Pages V2, Instagram V2), so changes affect all of them.

Link to Devin run: https://app.devin.ai/sessions/43cf7198b6104359a5bf79099ef93f18
Requested by: [email protected] (@ZdenekSrotyr)

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Merged into PR #23