Add platform definitions and default_platform support

[NeverBehave]

Summary

Addresses #4094 — adds platform support to the device type library.

• New platforms/ directory with <Manufacturer>/<slug>.yaml structure for platform definitions
• New schema/platformtype.json validating platform YAML (manufacturer, name, slug, description)
• default_platform field added to schema/devicetype.json as optional slug string
• Cross-validation ensures default_platform references an existing platform file
• Seed platforms: Arista EOS, Cisco IOS-XE, Cisco NX-OS, Juniper Junos, Palo Alto PAN-OS

Why a separate platforms/ directory?

A bare default_platform string would be a dangling reference — import tools can't resolve a platform that doesn't exist in the target NetBox instance. The platforms/ directory gives import tools a complete dependency chain: create platforms first, then device types.

Additionally, a device's platform doesn't always match its manufacturer (e.g., Edgecore switches running Cumulus Linux). Platforms need their own manufacturer field, which is why they live in dedicated files under their own manufacturer directory.

Test changes

• Platform files validated against platformtype.json schema
• Platform filename must match slug field
• default_platform in device types must reference an existing platforms/*/<slug>.yaml
• Duplicate platform slug detection across manufacturers
• Precomputed platform slug lookup for performance
• Device-only validations (verify_filename, validate_components) skipped for platforms

Test plan

• All 5 seed platform files pass schema validation
• Invalid slug pattern rejected by schema
• Missing required fields rejected by schema
• Extra fields rejected by additionalProperties: false
• Cross-validation correctly resolves valid platform slugs
• Cross-validation correctly rejects nonexistent platform slugs
• Filename=slug enforcement verified

[Copilot]

Pull request overview

Adds first-class platform definitions to the devicetype-library test + schema pipeline, enabling default_platform on device types and ensuring it resolves to an in-repo platform definition.

Changes:

• Introduces platforms/<Manufacturer>/<slug>.yaml definitions validated by a new schema/platformtype.json.
• Extends schema/devicetype.json to allow optional default_platform (platform slug).
• Updates test harness to validate platform files, enforce platform filename==slug, and cross-validate device type default_platform references.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_configuration.py	Registers the new platformtype.json schema for the platforms/ directory.
tests/device_types.py	Adds a PlatformType wrapper used by the shared validation test flow.
tests/definitions_test.py	Adds platform discovery + validation and default_platform cross-validation logic.
schema/platformtype.json	New JSON schema for platform YAML definitions.
schema/devicetype.json	Adds optional default_platform field with slug constraints.
platforms/Arista/arista-eos.yaml	Seed Arista EOS platform definition.
platforms/Cisco/cisco-ios-xe.yaml	Seed Cisco IOS-XE platform definition.
platforms/Cisco/cisco-nxos.yaml	Seed Cisco NX-OS platform definition.
platforms/Juniper/juniper-junos.yaml	Seed Juniper Junos platform definition.
platforms/Palo Alto/paloalto-panos.yaml	Seed Palo Alto PAN-OS platform definition.

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[Copilot]

KNOWN_PLATFORMS is built with the intent of detecting duplicate platform slugs, but duplicates are currently only surfaced when a device-type references default_platform. If two platform files share the same slug and no device-type references it, the tests will still pass. Consider adding an unconditional check (e.g., a small test that asserts every platform_slug maps to exactly one file) so duplicate slugs are always rejected.

Suggested change

	def test_unique_platform_slugs():
	"""
	Ensure each platform slug corresponds to exactly one platform definition file.
	This prevents silent duplicates when a platform slug is not referenced
	by any device-type via ``default_platform``.
	"""
	duplicates = {
	slug: files for slug, files in KNOWN_PLATFORMS.items() if len(files) > 1
	}
	assert not duplicates, (
	"Duplicate platform slugs detected:\n"
	+ "\n".join(
	f"{slug}: {', '.join(sorted(files))}"
	for slug, files in sorted(duplicates.items())
	)
	)