support manager domain fallback

[patmmccann]

Background

(RFC 4175) merged the ads.txt MANAGERDOMAIN fallback for adagents.json discovery into the spec and adcp repo. The validator now:

• Attempts /{publisher}/.well-known/adagents.json first
• On 404, consults /{publisher}/ads.txt for a MANAGERDOMAIN= directive
• If found, attempts /{manager}/.well-known/adagents.json
• Surfaces discovery_method: 'direct' | 'authoritative_location' | 'ads_txt_managerdomain' (required) and manager_domain?: string on AdAgentsValidationResult

adcp-client needs to implement this fallback logic and expose the new fields.

Implementation

• Implement the one-hop ads.txt MANAGERDOMAIN fallback in the validate_adagents path
• Add discovery_method: DiscoveryMethod (required, defaults to 'direct') and manager_domain?: string to AdAgentsValidationResult
• Export DiscoveryMethod type
• MANAGERDOMAIN= directive form only — comment form (# managerdomain=) must be rejected
• Ambiguous/duplicate MANAGERDOMAIN lines: last-wins (IAB-aligned per #4173 resolution)
• Manager domain 404 → terminal failure (not silent pass)

Test coverage

• Direct path sets discovery_method: 'direct'
• URL-reference path sets discovery_method: 'authoritative_location'
• Manager domain path sets discovery_method: 'ads_txt_managerdomain' and populates manager_domain
• Comment-form # managerdomain= is NOT followed
• Duplicate MANAGERDOMAIN lines → last entry wins
• Manager domain 404 → validation failure

Refs: adcontextprotocol/adcp#4173, adcontextprotocol/adcp#4175