Feat: Support gateway in model defaults

[josephfinlayson]

Description

Multi-project contexts can already load models from independently configured projects into one graph and shared state store. However, a model without an explicit gateway currently inherits the one context-selected gateway, so every model in a secondary project must repeat the same gateway property.

This adds model_defaults.gateway as a project-level default for managed SQL, Python, and seed models:

model_defaults:
  dialect: duckdb
  gateway: project_gateway

The default is resolved before model name qualification, so the model receives the selected gateway's default catalog and gateway variables. An explicit gateway in a SQL MODEL block or Python decorator still takes precedence.

Plain Python decorator gateway strings are treated as literal gateway names, including names with characters such as hyphens. Macro-prefixed Python gateway values retain blueprint rendering, as do SQL MODEL gateway expressions.

External models intentionally remain gateway-neutral unless their YAML row specifies a gateway. For external models, gateway selects an alternate source definition rather than an execution default.

No behavior changes when model_defaults.gateway is omitted.

Prior art

This builds on:

• Feat!: Add multiple engine project support #3394, which introduced model-level multi-engine routing.
• Feat: Add model defaults support per gateway configuration #3888, which introduced gateway-specific model defaults.
• Feat!: Add support for multiple virtual layers controlled by gateway #4101, which introduced gateway-managed virtual layers.
• Add model defaults at the subfolder level within the models/ directory #4996, which discusses broader default scoping but not project-level gateway routing.

Test plan

• Added SQL model coverage for project defaults, explicit overrides, gateway variables, gateway-specific default catalogs, and hyphenated gateway names.
• Added Python model and blueprint coverage, including literal hyphenated gateways and macro-prefixed blueprint gateways.
• Added external-model regression coverage.
• Added a two-project Context integration test proving each project uses its own default gateway, catalog, and variables while explicit overrides still win.
• make style
• make fast-test (2,777 passed across the fast, isolated, registry-isolation, and dialect-isolation phases)
• Focused model/config/schema-loader plus new slow integration test (412 passed)

Checklist

• I have run make style and fixed any issues.
• I have added tests for my changes.
• All existing tests pass (make fast-test).
• My commits are signed off per the DCO.

[cmgoffena13]

Looks good to me, let me fix the jobs and see if anything comes up. I have a PR to fix the failing dbt test and the other python tests should succeed on retries (found a UTC boundary bug in there). Once all the CI jobs are green I'll merge.