fix(buzz): the buzz CLI subprocess env leaks the launch profile's residue

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Summary

_exec_buzz — the sole spawn point for the external buzz CLI binary — built its child env with a bare os.environ.copy(). Under gateway.multiplex_profiles, one process serves several profiles and raw os.environ holds the launch profile's YAML-to-env bridge output plus its .env residue and bridged TERMINAL_* settings. A served secondary profile's buzz invocation inherited all of it, including a HERMES_HOME that pointed at the wrong profile's home.

#99427 (salvaging #98748) already fixed the read side of this class for Buzz: BuzzAdapter's own config resolution (relay/channels/credentials/allowed users) now goes through _scoped_platform_setting() so a secondary profile's PlatformConfig wins over stale env. But that PR's diff never touched _exec_buzz's env construction (confirmed via gh pr diff — the env = os.environ.copy() line is an unchanged context line in that PR) — the BUZZ_RELAY_URL/BUZZ_PRIVATE_KEY/BUZZ_AUTH_TAG values it injects were already correctly scoped by the caller, so the leak was easy to miss: every other ambient variable the launch profile's environment carries (HERMES_HOME foremost) still rode along unscoped.

The same #111617 multiplex retrofit that fixed 9 other subprocess/background-thread spawn sites this window established served_profile_child_env() for exactly this shape (a2a's forward-to-profile subprocess, browser_tool's driver spawn, etc.) — this call site was missed.

Fix

Switches _exec_buzz to served_profile_child_env(inherit_credentials=False), mirroring tools/browser_tool.py's identical external-CLI-driver pattern: the target profile's home is pinned and the launch profile's residue is dropped, while the buzz CLI's own credentials (relay_url/private_key/auth_tag) keep arriving explicitly via the function's own parameters, unchanged.

Testing

• Added TestExecBuzzChildEnv::test_served_profile_exec_drops_launch_profile_residue: sets up a launch home with a TERMINAL_ENV bridge var, arms multiplex + a served-profile home override, captures the env passed to a monkeypatched asyncio.create_subprocess_exec, and asserts HERMES_HOME is the served profile's home (not the launch profile's), TERMINAL_ENV residue is absent, and the caller-supplied BUZZ_RELAY_URL/BUZZ_PRIVATE_KEY still win.
• Mutation-verified: reverted just the production line, confirmed the new test fails (HERMES_HOME leaked the launch profile's path), restored the fix, confirmed the full tests/gateway/test_buzz_adapter.py suite passes (187/187).
• Also ran tests/agent/test_subprocess_env_guard.py (2/2) to confirm no regression in the sibling guard.

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