Add model_validator to CardDelegationConfig to enforce valid field combinations

[Copilot]

Description

CardDelegationConfig allowed constructing an empty or partially-populated instance (all fields optional), deferring validation failures to the backend. This adds a Pydantic model_validator that enforces valid combinations at construction time.

Three valid modes (exactly one must be satisfied):

• delegation_id — reuse an existing delegation
• card_id — reuse an enrolled card; optionally override spending_limit_cents, duration_secs, currency, etc.
• provider_payment_method_id + spending_limit_cents + duration_secs — create a brand-new delegation (all three required)

An empty config or a partial new-delegation config now raises ValueError immediately:

# Raises: "CardDelegationConfig requires at least one of: ..."
CardDelegationConfig()

# Raises: missing field listed in error
CardDelegationConfig(provider_payment_method_id="pm_123", spending_limit_cents=5000)

# Valid — reuse card with optional overrides
CardDelegationConfig(card_id="pm-card-abc", spending_limit_cents=5000, duration_secs=3600)

Docstring updated to clearly document all three modes and their field requirements. 10 new unit tests cover valid and invalid combinations.

Is this PR related with an open issue?

Related to PR #147 — addresses review feedback

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Follows the code style of this project.
• Tests Cover Changes
• Documentation

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