Add upsert_resource() and external_id support

[loookashow]

Summary

• Add upsert_resource() method to ManagementClient and AsyncManagementClient — creates or updates a resource by external_id in a single call (PUT /folders/:folder/resources/?external_id=<value>)
• Add optional external_id parameter to create_resource() — passes it in the request body so the API assigns the identifier on creation
• Add external_id field to ResourceSummary model

Changes

SDK (src/foxnose_sdk/management/)

• models.py — external_id: str | None = None on ResourceSummary
• client.py — upsert_resource() and external_id param on create_resource() for both sync and async clients

Tests (tests/)

• 16 new unit tests (11 sync, 5 async) covering upsert, external_id in create, payload immutability, backward compatibility

Docs (docs/)

• management-client.md — Create Resource updated, new Upsert Resource section
• examples.md — new Upsert section with usage examples
• changelog.md — v0.3.0 release notes

Test plan

• All tests pass (pytest tests/ -v)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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