Improve error cause logging

[chiragjn]

Note

Low Risk
Observability and user-facing error text only; no auth, data, or sandbox execution semantics changed beyond clearer messages and logging.

Overview
Improves diagnostics when TrueFoundry sandbox HTTP calls fail, so operators and agent steps see actionable detail instead of generic undici fetch failed text.

describeUnknownError and extractErrorLogFields now walk nested Error.cause (with cycle detection and de-duplication when the head message already contains the tail). That propagates to other callers (e.g. agent turn errors, LLM errors), not only the sandbox provider.

TFYSandboxProvider threads the exec / upload URL into error strings and structured logs, uses describeUnknownError for exec and upload network failures (upload rethrows with cause preserved), and adds info logs when sandbox file uploads start and finish (sandbox id, path, byte count).

OpenAPI metadata is bumped to 0.2.1; a patch changeset documents the @truefoundry/trueforge-core release note.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 0c875ba

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@truefoundry/trueforge-core	Patch
@truefoundry/trueforge	Patch

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