Principal aware auth for agent tool call

[omrishiv]

Add principal-aware authorization across agent-management and delegation paths.

• Scope agent creation and listing to the authenticated caller.
• Prevent delegation to inaccessible agents and recheck access on resume.
• Hide private agents from A2A discovery and exports.
• Require owner/admin authorization for deletion.
• Use owner-qualified deletion to prevent same-name cross-tenant removal.
• Add cross-tenant isolation regression coverage.

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