Scope the multiplexed-session emulator workaround safely

[apstndb]

Part of #591.

Overview

init() unconditionally sets GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW=false as a global process-wide override.

Evidence

• internal/mycli/app.go:73-77

Impact

The emulator workaround is applied even for real Cloud Spanner traffic, and users cannot opt back in by setting the environment variable themselves.

Acceptance criteria

• The workaround no longer overwrites an explicit user-provided environment value.
• The workaround is scoped narrowly enough to avoid affecting non-emulator usage, or the rationale for any wider scope is explicit and documented.
• Tests cover the chosen precedence behavior.