Honor profile.authType in default credential chain and delegate name()

[parthban-db]

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• stack/bugbash-bug6-authtype-name [Files changed]

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Summary

The default credential chain now honors profile.authType, and DefaultCredentials.name() reports the strategy that actually resolved instead of always returning default.

Why

The chain previously tried strategies in a fixed order (PAT, OAuth M2M, Databricks CLI) and stopped at the first applicable one, ignoring profile.authType even though it is parsed from auth_type / DATABRICKS_AUTH_TYPE. A profile that set auth_type = databricks-cli but also carried a token would silently use PAT, with no way to pin a method. Separately, name() was hardcoded to default, so logging and future User-Agent auth= telemetry could never tell which strategy won.

AI Generated

What changed

• Strategy is now an interface {name, configure} instead of a bare function, so the chain can match a strategy against authType by name (pat, oauth-m2m, databricks-cli) before building it. Converted patStrategy, m2mStrategy, and u2mStrategy to this shape with unchanged config logic.
• When authType is set, the chain configures only the matching strategy with no fallback: it throws NO_AUTH_CONFIGURED if that strategy cannot configure, and the new AUTH_TYPE_NOT_FOUND code if no strategy matches. When authType is unset, ordered fall-through behavior is unchanged.
• name() returns default before resolution and the resolved strategy's name afterward.
• These types are internal (not exported from the package barrel), so this is not a public API change.
• Mirrors the Go SDK's DefaultCredentials (config/auth_default.go); the JS strategy set is smaller, so fewer authType names are accepted — an existing scope difference. Composing the User-Agent auth= segment is out of scope (JS does not emit one yet).

Validated: npm run build, npm test, npm run test:browser, npm run typecheck, npm run lint, and npm run format:check all pass for @databricks/sdk-auth.

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