feat(types): add budget field to IntentMandate

[ravyg]

Adds an optional budget: PaymentCurrencyAmount field to IntentMandate in both Python and Go, giving agents a machine-readable spend ceiling to enforce when fulfilling a purchase intent.

• Python: src/ap2/types/mandate.py -- new field + import
• Go: samples/go/pkg/ap2/types/mandate.go -- new field with omitempty
• Tests: 6 Python tests + 5 Go tests (optional, set, JSON round-trip, absent-from-JSON)
• Docs: Section 4.1 Intent Mandate field list updated

Closes #133

Description

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial enhancement to the IntentMandate type by adding an optional budget field. This new field provides a mechanism for agents to define and enforce a maximum spend limit for a given purchase intent, thereby improving control and predictability in automated commerce scenarios. The change is implemented across both Python and Go language bindings, accompanied by thorough testing and updated documentation.

Highlights

• New Feature: Budget Field: An optional budget field of type PaymentCurrencyAmount has been added to the IntentMandate type in both Python and Go implementations. This field allows agents to enforce a machine-readable spend ceiling for purchase intents.
• Cross-Language Implementation: The budget field was added to IntentMandate in src/ap2/types/mandate.py (Python) and samples/go/pkg/ap2/types/mandate.go (Go), ensuring consistency across language bindings.
• Comprehensive Testing: Extensive tests were added for both Python and Go, covering scenarios such as the field's optionality, successful setting of a budget, correct JSON serialization and deserialization, and proper omission from JSON output when the field is not set.
• Documentation Update: The docs/specification.md file was updated to reflect the addition of the new Budget field within the Intent Mandate section, providing clear guidance on its purpose and usage.

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[gemini-code-assist]

Code Review

This pull request introduces a new 'Budget' field to the IntentMandate type, allowing for an optional maximum total spend ceiling for AI agents. The change is implemented in both Go and Python, including updates to the specification documentation and comprehensive unit tests covering optionality, setting, JSON serialization, and round-trip functionality. The only feedback provided suggests using a standard apostrophe for consistency in the markdown documentation.

[gemini-code-assist]

Consider using a standard apostrophe ' instead of the right single quotation mark ’ for consistency in markdown files, unless there's a specific style guide dictating otherwise. While ’ is typographically correct, '' is more common in plain text and code contexts.

Suggested change

	- Time-to-Live (TTL): An expiration time for the mandate’s validity.
	- Time-to-Live (TTL): An expiration time for the mandate's validity.