Fix include specs collision

[malka4325]

Summary & Motivation

Fix collision detection for Asset + AssetSpec with the same AssetKey

Currently, when loading modules with include_specs=True, Dagster raises a
duplicate asset error if an Asset and an AssetSpec share the same AssetKey.
This prevents horizontal imports and proper use of AssetSpecs across files.

This change allows a single executable Asset to coexist with one or more
AssetSpecs for the same key without raising an error, while still raising
errors for duplicate executable Assets.

How I Tested These Changes

Added unit tests covering:

• Loading an Asset + AssetSpec with the same key does not raise an error.
• Loading two executable Assets with the same key still raises DagsterInvalidDefinitionError.
• Ran all existing Dagster tests locally to ensure nothing else is broken.

Changelog

• Fix collision detection in load_assets_from_modules when include_specs=True.
• Add tests for Asset + AssetSpec coexistence.

[malka4325]

@cmpadden can you unblock the buildkite?

[cmpadden]

Thank you @malka4325.

There is a potential failure in this implementation (see test below) in that the new collision logic only ignores specs when len(asset_defs) == 1. In this test, the same AssetsDefinition object is imported into two modules, so asset_defs has length 2 but only one distinct id. That passes the "multiple distinct defs" check, but because len(asset_defs) != 1, the code does not skip specs. It then compares the two AssetSpec objects (they differ due to description) and raises a collision, even though an AssetsDefinition exists and should make specs irrelevant.

@pytest.mark.xfail(
    reason=(
        "Asset specs should be ignored when an AssetsDefinition exists, even if the "
        "AssetsDefinition is imported in multiple modules."
    ),
    strict=True,
)
def test_assets_def_imported_twice_with_conflicting_specs_does_not_raise():
    mod1 = types.ModuleType("mod1")
    mod2 = types.ModuleType("mod2")

    @dg.asset
    def asset1():
        return 1

    mod1.asset1 = asset1
    mod2.asset1 = asset1
    mod1.asset1_spec = dg.AssetSpec("asset1", description="mod1")
    mod2.asset1_spec = dg.AssetSpec("asset1", description="mod2")

    dg.load_assets_from_modules([mod1, mod2], include_specs=True)