chore(graph-utils): export GraphClient base class

[TalZaccai]

Make the GraphClient base class available alongside CalendarClient / MailClient (which already extend it) so downstream agents can build new Microsoft Graph–backed clients without copying the auth/transport plumbing.

Change

ts/packages/agents/agentUtils/graphUtils/src/index.ts — add GraphClient to the existing named re-export from ./graphClient.js.

Risk

Additive-only. No existing consumer breaks because the export was previously narrower; all current callers use the more specific subclasses or ErrorResponse.

Test

• pnpm --filter graph-utils build ✓
• pnpm --filter graph-utils prettier ✓
• Verified GraphClient shows up in dist/index.d.ts after the change.

[Copilot]

Pull request overview

This PR broadens the public API surface of the graph-utils package by exporting the shared GraphClient base class from the package entrypoint, enabling downstream agents to build additional Microsoft Graph–backed clients without duplicating auth/transport logic.

Changes:

• Re-export GraphClient (alongside ErrorResponse) from src/index.ts so it becomes available via the package root export.

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[TalZaccai]

Superseded by a combined PR that adds the actual motivation for exporting GraphClient: a GraphClientOptions parameter so subclasses can target a different AAD app (custom env-var prefix, persisted auth-record filename, token-cache name, and OAuth scopes). Re-opening as a single PR from talzacc/graph-client-options-refactor since the export alone has no in-tree consumer.