[mongo] Fall back to configured host when serverStatus hostname resolution fails

[pierreln-dd]

What does this PR do?

When serverStatus() fails (e.g. transient connection error, Atlas Private Link connectivity issue on first check dispatch), the hostname property in MongoInterface silently returns None. This None propagates into _refresh_metadata where set_metadata('resolved_hostname', None) is called without a guard, causing the C extension binding to raise TypeError: argument 3 must be str, not None.

This PR fixes both layers:

1. api.py: Fall back to self._config.hosts[0] when serverStatus() fails, mirroring how the Postgres integration falls back to the configured DB host when server-side hostname resolution fails. The fallback is not cached in __hostname so serverStatus() is retried on subsequent check runs.

2. mongo.py: Add a None guard before set_metadata('resolved_hostname', ...) as defence-in-depth for any future code path where hostname resolution yields None.

Motivation

SDBM-2618: Customer's MongoDB integration running as a cluster check (cluster_check: true) against an Atlas Private Link endpoint fails with TypeError: argument 3 must be str, not None on every check run in production. The cluster check runner node cannot reach serverStatus() on initial dispatch, hostname returns None, and the unchecked set_metadata call crashes. Adding reported_database_hostname to the config is the confirmed workaround — this PR makes the integration resilient without requiring that config override.

Root cause is present since Agent 7.56.0 (mongo check 6.7.0) when database_autodiscovery was introduced.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[datadog-official]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 25.00%
• Overall Coverage: 91.55%

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 083d1db | Docs | Datadog PR Page | Give us feedback!}

[dd-octo-sts]

Validation Report

All 20 validations passed.

Show details

Validation	Description	Status
agent-reqs	Verify check versions match the Agent requirements file	✅
ci	Validate CI configuration and Codecov settings	✅
codeowners	Validate every integration has a CODEOWNERS entry	✅
config	Validate default configuration files against spec.yaml	✅
dep	Verify dependency pins are consistent and Agent-compatible	✅
http	Validate integrations use the HTTP wrapper correctly	✅
imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

View full run

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.00%. Comparing base (03b790e) to head (083d1db).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.