Fixes #10965

This change updates the Bigtable client to use the `passthrough:///`
scheme when connecting to the emulator via `BIGTABLE_EMULATOR_HOST`.

This ensures consistent behavior with other Google Cloud Go clients
(e.g., Spanner #10947)
and avoids potential gRPC name resolution issues by forcing the
passthrough resolver. It handles `BIGTABLE_EMULATOR_HOST` values with or
without an existing scheme (e.g. `localhost:9000` or
`http://localhost:9000`).

…gsValidation

Without SPANNER_EMULATOR_HOST set, the Spanner client library doesn't add
the passthrough:/// prefix or skip dial settings validation. This causes
~100x slowdown (~1s per client creation) because the new authentication
library hits a slow auth path. Since spanemuboost connects via emu.URI()
without setting the env var, the fix from googleapis/google-cloud-go#10947
was being bypassed.

Add passthrough:/// prefix to endpoint and internaloption.SkipDialSettingsValidation()
to both ClientOptions() and defaultClientOpts().

Fixes #7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

)

Without SPANNER_EMULATOR_HOST set, the Spanner client library doesn't add
the passthrough:/// prefix or skip dial settings validation. Since spanemuboost
connects via emu.URI() without setting the env var, the fix from
googleapis/google-cloud-go#10947 is bypassed.

Currently the auth layer uses grpc.DialContext (passthrough by default), so
this is not actively triggered, but it will regress when the auth layer
migrates to grpc.NewClient (dns resolver by default).

Changes:
- Add passthrough:/// prefix to endpoint in defaultClientOpts()
- Add internaloption.SkipDialSettingsValidation() (required by passthrough:///)
- Delegate Emulator.ClientOptions() to defaultClientOpts() to avoid duplication
- Update test assertion from 3 to 4 options

This aligns with the pattern used by Spanner, Bigtable, and Datastore SDKs
for their emulator paths.

Fixes #7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>