chore(examples): upgrade Better Auth to 1.7 - #2446

Summary

• upgrade the two in-memory Better Auth examples to better-auth and @better-auth/oauth-provider 1.7.2
• replace the removed validAudiences configuration with resource-bound OAuth configuration
• assign the MCP resource to dynamically registered clients through default and allowed resource policy
• remove the 1.6-only warning suppression and update the workspace lockfile
• retain exactOptionalPropertyTypes with a localized workaround for upstream Better Auth issue #10213

This PR is stacked on #2426 and resolves the remaining @better-auth/oauth-provider Dependabot alert (GHSA-p2fr-6hmx-4528). No database migration is needed because both examples intentionally use in-memory storage.

Testing

• pnpm install --frozen-lockfile
• built packages/client and packages/server
• typechecked both Better Auth example packages
• ESLint and Prettier checks on all changed source/config files
• vitest run tests/oauth-direct-providers.test.ts (17 tests)
• manually smoke-tested OAuth and OpenID discovery, dynamic native-client registration, default resource assignment, and the resource-bound authorization redirect

Summary by cubic

Upgrades the Better Auth examples to 1.7.2 and updates the workspace override range, adapting the OAuth provider plugin to Better Auth's new resources config API.

Migration

• Replace validAudiences with resources, clientRegistrationDefaultResources, and clientRegistrationAllowedResources.
• Remove silenceWarnings; it no longer exists in 1.7.

^{Written for commit b5448c6. Summary will update on new commits.}