.NET: Scope argument-based standing approvals correctly in ToolApprovalAgent

[rogerbarreto]

Fixes #6486

Scopes argument-based standing approvals correctly in ToolApprovalAgent. An argument-scoped standing approval (the "always approve with exact arguments" path) now records an empty argument set instead of null when the approved call has no arguments, so it matches only future no-argument calls. null stays reserved exclusively for tool-level approvals, keeping the two scopes distinct. Aligns .NET behavior with the existing Python harness.

Changes

• ToolApprovalAgent.SerializeArguments returns a non-null (empty) dictionary for empty/null args.
• Clarified XML docs on ToolApprovalRule.Arguments semantics.
• Regression tests: no-argument standing-approval flow, MatchesRule argument scoping, empty-args rule serialization.

Validation

• Build: 0 warnings / 0 errors.
• Tests: 1652 (net10.0) + 1542 (net472) pass.
• dotnet format clean on changed projects.

[Copilot]

Pull request overview

This PR fixes a scoping bug in the .NET tool-approval harness where an “always approve with exact arguments” standing approval for a no-argument tool call was being stored with null arguments, unintentionally widening it to a tool-level approval. The change ensures no-argument argument-scoped approvals are persisted as an empty argument set ({}), preserving the intended narrow scope and aligning behavior with the existing Python harness.

Changes:

• Update ToolApprovalAgent.SerializeArguments to always return a non-null dictionary (empty when the call has no arguments) so argument-scoped rules never serialize as tool-level rules.
• Clarify ToolApprovalRule.Arguments XML documentation to explicitly define null vs empty-dictionary semantics.
• Add regression tests covering empty-args rule serialization round-trips and empty-args matching/auto-approval behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalAgent.cs	Ensures argument serialization for standing approvals returns {} (not null) for no-arg calls, preserving argument-scoped rule semantics.
dotnet/src/Microsoft.Agents.AI/Harness/ToolApproval/ToolApprovalRule.cs	Documents the intended distinction: null = tool-level; empty dict = no-argument exact-match scope.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalAgentTests.cs	Adds regression tests verifying empty-args exact-approval does not auto-approve later argument-bearing calls, but does auto-approve later no-arg calls.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/ToolApproval/ToolApprovalRuleTests.cs	Adds a round-trip serialization test proving empty-args rules persist as non-null empty dictionaries.

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 94% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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