feat(a2a): add plus api token auth

[greysonlalonde]

Note

Medium Risk
Adds new server auth scheme surface area and new config/error codes that affect how authenticated-only agent card data may be advertised/validated. While EnterpriseTokenAuth is currently a stub, changes in auth-related modules can impact request authorization behavior once wired up.

Overview
Adds a new A2A server auth scheme entry point, EnterpriseTokenAuth, intended for validating bearer tokens against PlusAPI, and exports it via crewai.a2a.auth.

Extends A2A server configuration to support authenticated-only metadata by adding extended_skills, and introduces a dedicated protocol error (AUTHENTICATED_EXTENDED_CARD_NOT_CONFIGURED) for misconfigured authenticated extended card usage.

Also adds a general-purpose slugify() helper in string_utils and makes a small typing/import cleanup in OAuth2 JWT/introspection auth (httpx import and jwt decode options typing).

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 3 total unresolved issues (including 2 from previous reviews).

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