This change:
1. Updates `DataSourceContractTest` to allow multiple "not found"
resources, and to include additional info (e.g. headers) on them.
2. Updates the contract test to assert that `DataSource.getUri()`
returns the expected (non-null) value for "not found" resources
between the failed `open()` call and a subsequent `close()` call.
The `DataSource` is 'open' at this point (since it needs to be
'closed' later), so `getUri()` must return non-null.
* This change also fixes some implementations to comply with this
contract. It also renames some imprecisely named `opened`
booleans that **don't** track whether the `DataSource` is open
or not.
3. Updates the contract test assertions to enforce that
`DataSource.getResponseHeaders()` returns any headers associated
with the 'not found' resource.
4. Configures `HttpDataSourceTestEnv` to provide both 404 and "server
not found" resources, with the former having expected headers
associated with it.
PiperOrigin-RevId: 689316121
