Import gap: `use_latest_adapter` is not set for imported bigquery_v1 connections, causing adapter-change errors and compounding timeout_seconds bug

Addendum: import/read does not set `use_latest_adapter` for bigquery_v1 connections — causes false 'Adapter version cannot be changed' errors and compounds timeout_seconds issue.

When importing a BigQuery v1 global connection (either via CLI `terraform import` or config-driven `import {}`), the provider's read/import path populates adapter_version = "bigquery_v1" but does not set `bigquery.use_latest_adapter` in state. This means state has `use_latest_adapter = null` while the connection is v1. If the config has `use_latest_adapter = true`, the provider's ModifyPlan guard treats this as a user-requested adapter change and errors with "Adapter version cannot be changed". If the config omits the flag, the plan treats the resource as v0, injects the v0-only default `timeout_seconds = 300` into the plan, and the Update path sends it to the API, which rejects it for BigqueryConnectionV1.

Proposed remediation steps to include in the same issue:

1. In readGeneric (pkg/framework/objects/global_connection/common.go), when reading a BigQuery connection, derive and set `state.BigQueryConfig.UseLatestAdapter` based on the adapter_version returned by the API (e.g., true if adapter_version starts with "bigquery_v1" or equals the latest adapter). This allows imported v1 connections to round-trip and avoids the ModifyPlan false positive.

2. In the Update path (pkg/framework/objects/global_connection/resource.go), guard sending `TimeoutSeconds` to the API: only include `warehouseConfigChanges.TimeoutSeconds` when the plan indicates the legacy adapter (use_latest_adapter == false). The Create path already does adapter-aware handling; mirror that behavior in Update.

3. Add regression tests for:
- Importing a bigquery_v1 connection and then running a no-op plan/apply (should be no adapter-change errors).
- Updating a bigquery_v1 connection with no timeout_seconds set should NOT PATCH `timeout_seconds`.

Rationale: these two changes together ensure imported v1 connections behave correctly in Terraform, and prevent the provider from sending v0-only fields to v1 API objects. They resolve both the immediate failure mode and prevent future surprises for other users using import blocks.