.NET: Improve string parsing in declarative workflows

[Peter Ibekwe (peibekwe)]

Motivation & Context

Improve string parsing in declarative workflows.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[github-actions]

Automated Code Review

Reviewers: 2 | Confidence: 84% | Result: All clear

Reviewed: Correctness, Test Coverage

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Automated review by peibekwe's agents

[Copilot]

Pull request overview

This PR improves how declarative workflow code extracts JSON from fenced code blocks by replacing the previous regex-based implementation with a linear scan approach, and expands unit coverage for common LLM output shapes (surrounding text, Windows line endings, multiple fenced blocks, and malformed input).

Changes:

• Replaced TrimJsonDelimiter regex parsing with a linear scan to avoid regex backtracking on malformed input.
• Added new TrimJsonDelimiter tests for surrounding text, Windows line endings, first-block selection, qualifier edge cases, and unterminated fences.
• Added a conversion test ensuring fenced JSON strings can be converted into record types.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Extensions/StringExtensions.cs	Reworks fenced-JSON trimming to a linear scan implementation (no regex).
dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/Extensions/StringExtensionsTests.cs	Adds coverage for additional fenced-JSON trimming scenarios and edge cases.
dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/Extensions/ObjectExtensionsTests.cs	Adds a test validating type conversion from fenced JSON input.

[github-actions]

Automated Code Review

Reviewers: 2 | Confidence: 93% | Result: All clear

Reviewed: Correctness, Test Coverage

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Automated review by peibekwe's agents