fix(errors): extract API error body from GenericOpenAPIError

[maruina]

Problem

datadog-api-client-go consumes http.Response.Body during deserialization and stores the raw bytes in datadog.GenericOpenAPIError. formatAPIError() re-read http.Response.Body, which was always empty by that point. Users saw only:

Error: failed to query metrics: 400 Bad Request (status: 400)

No indication of what went wrong.

Fix

Extract the response body from GenericOpenAPIError.Body() instead of re-reading the drained http.Response.Body. API errors now surface the actual server response:

Error: failed to query metrics: 400 Bad Request
Status: 400
API Response: {"errors":["Error parsing query: unable to parse avg:this.metric.does.not.exist.at:
Rule 'scope_expr' didn't match at '' (line 1, column 34)."]}

Changes

• Add extractAPIErrorBody() — unwraps GenericOpenAPIError and returns the stored body (cmd/root.go:264)
• Call extractAPIErrorBody() from formatAPIError() to include the body in all API error paths (cmd/root.go:291)
• Apply the same extraction in cmd/logs_simple.go and cmd/metrics.go for direct error handling

Testing

• Unit tests for extractAPIErrorBody: body present, empty body, nil body, non-API error, nil error (cmd/root_test.go:323)
• Integration test: formatAPIError includes the response body in output (cmd/root_test.go:375)
• Manual: pup metrics query --query="avg:invalid.metric" --from="1h" shows the full API parse error

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🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 81.7%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/alias.go:26: 100.0%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:28: 96.0%

## Summary

total:								(statements)		81.7%

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📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit cab8890}