Handle multi-block sampling responses in SampleStructuredAsync

[m-nash]

Summary

Fixes a sampling failure where monitoring a PR errors with Sampling failed to explain comment after 2 attempts / 'e' is invalid after a value.

The Copilot sampling backend can return the assistant answer across multiple content blocks — sometimes a truncated false-start block followed by the complete answer. ExtractText joined all blocks with \n, fusing them into invalid JSON (e.g. {"explanation": "...ind\n{"explanation": ...}), which then failed to deserialize.

Changes

• SampleStructuredAsync now tries each content block individually (last→first) — the final block is usually the complete answer — then falls back to the joined text for providers that split one object across blocks. First candidate that deserializes into T wins.
• Extracted SampleRawAsync, ExtractTextBlocks, and a TryDeserialize<T> helper. Added blocks=N diagnostic logging.
• SampleTextAsync / ExtractText (plain-text path) keep the existing join behavior.
• FakeSamplingMcpServer gains a params string[] constructor to simulate multi-block responses.

Tests

Three new tests in SamplingHelperTests:

• MultipleBlocks_UsesCompleteBlock — truncated first + complete second (the exact failure)
• MultipleBlocks_CompleteBlockFirst — either ordering works
• MultipleBlocks_OneObjectSplitAcrossBlocks_FallsBackToJoin — back-compat

All 350 tests pass; the two multi-block tests are verified to fail against the previous code. Also validated live locally via a dev build before opening this PR.

[Copilot]

Pull request overview

This PR fixes a structured-sampling parsing failure when the MCP sampling backend returns assistant output across multiple content blocks (e.g., a truncated false start followed by a complete JSON answer). The update changes SampleStructuredAsync to try deserializing each block individually (preferring the last block) before falling back to deserializing the joined text, preventing invalid JSON formed by naive concatenation.

Changes:

• Update SampleStructuredAsync to attempt JSON deserialization per content block (last→first), then fall back to joined blocks when needed.
• Refactor sampling helpers (SampleRawAsync, ExtractTextBlocks, TryDeserialize<T>) and add blocks=N diagnostics.
• Add tests covering multi-block behaviors and extend FakeSamplingMcpServer to simulate multi-block responses.

Reviewed changes

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

File	Description
PrCopilot/tests/PrCopilot.Tests/SamplingHelperTests.cs	Adds coverage for multi-block structured sampling responses (complete block preference + join fallback).
PrCopilot/tests/PrCopilot.Tests/FakeSamplingMcpServer.cs	Enhances the test MCP server to emit multiple TextContentBlocks via a params string[] constructor.
PrCopilot/src/PrCopilot/Tools/SamplingHelper.cs	Implements block-aware structured parsing with per-block deserialization attempts and a join fallback for split objects.

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