fix(boxel-cli): Extract fetch error.cause on publish/unpublish failures

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While working on #4897, there were unhelpful fetch failed errors like here when publishing wasn’t working. This extracts more detail from error.cause to help clarify failures.

[Copilot]

Pull request overview

This PR improves diagnostics in @cardstack/boxel-cli when realm publish/unpublish operations fail due to Node fetch() reporting only TypeError: fetch failed, by extracting and surfacing the underlying transport error stored on error.cause.

Changes:

• Include error.cause details in readiness polling timeout errors during realm publish.
• Log error.cause in the realm publish CLI error path to provide actionable transport failure details.
• Inline error.cause details into the returned error string for realm unpublish failures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
packages/boxel-cli/src/commands/realm/publish.ts	Adds a helper to summarize fetch errors (including error.cause) for readiness polling and logs causes on CLI failures.
packages/boxel-cli/src/commands/realm/unpublish.ts	Enhances unpublish failure reporting by appending error.cause details to the returned error message.

Comments suppressed due to low confidence (1)

packages/boxel-cli/src/commands/realm/publish.ts:209

• This change introduces new user-facing error formatting (including error.cause) for readiness polling timeouts, but there’s no automated coverage to prevent regressions in the output. Consider adding a unit test for describeFetchError() (and/or a small test that simulates a fetch error with an Error cause) so the thrown timeout message reliably includes the underlying transport detail.

      }
      lastError = `HTTP ${response.status}`;
    } catch (error) {
      lastError = describeFetchError(error);
    }
    let remaining = timeoutMs - (Date.now() - startedAt);
    if (remaining <= 0) break;
    await new Promise((r) =>
      setTimeout(r, Math.min(READINESS_POLL_INTERVAL_MS, remaining)),
    );
  }

  throw new Error(
    `Timed out after ${timeoutMs}ms waiting for ${publishedRealmURL} to pass readiness check${
      lastError ? `: ${lastError}` : ''
    }`,
  );

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