fix(contrib/mcp-go): Report structured tool call errors as error spans

[jboolean]

What does this PR do?

Fix for spans not marked as errors for tool calls that resulted in an error response.

mcp-go has two ways for a tool to return an error as documented here: https://mcp-go.dev/servers/tools#error-results

We were not handling the "structured error information" case where IsError is part of the response. This is part of the MCP Spec.

Motivation

Traces like this not marked as errors.

Closes https://datadoghq.atlassian.net/browse/MLOB-4640

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[jboolean]

• fix(contrib/mcp-go): Report structured tool call errors as error spans #4158 👈 (View in Graphite)
• feat(contrib/mcp-go): Simplified way to add tracing #4122 : 1 other dependent PR (#4123 )
• feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans #4101 : 2 other dependent PRs (#4106 , #4115 )
• feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100
• main

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-20 21:36:35

Comparing candidate commit 50ff2b2 in PR branch jb/contrib-mcp-go-tool-is-error with baseline commit a01f947 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 3 metrics, 0 unstable metrics.

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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