proxy: domain-aware session handshakes

[a2105z]

Summary

• Scope HTTP proxy StartSession to the domains each command actually needs instead of always handshaking both VCSEC and Infotainment.
• Skip StartSession for wake_up (Fleet REST wake; no signed session required).
• Keep unclassified / future commands on the previous both-domain behavior.

Closes #468.
Also addresses proxy impact of #450 and #285 for VCSEC-only commands (lock/unlock/trunk/tonneau/remote drive) when Infotainment is unreachable.

What was happening

liveVehicle.StartSession always called Vehicle.StartSession(ctx, nil), which expands to both domains. If Infotainment handshake failed, VCSEC-only commands never ran.

What happens with this change

Command class	Domains started
door_lock, door_unlock, actuate_trunk, tonneau, remote_start_drive	VCSEC only
Climate / charging / media / windows / etc.	Infotainment only
wake_up	none
Unknown commands	both (unchanged default)

Risks

• Misclassification: unknown commands default to both domains; map is derived from pkg/vehicle (executeRKEAction / executeClosureAction vs executeCarServerAction).
• Incidental Infotainment warm from lock requests no longer happens; clients that need Infotainment should call an Infotainment command or wake explicitly.

Test plan

• go test ./pkg/proxy/ -count=1
• door_lock starts VCSEC only
• auto_conditioning_start starts Infotainment only
• wake_up does not call StartSession
• Maintainer review of domain classification table

@sethterashima @patrickdemers6 @agbpatro — please take a look when you have a chance.

[a2105z]

@sethterashima @patrickdemers6 @agbpatro — review request.

Issues addressed

• Closes feat: Domain-aware session handshakes in HTTP proxy #468
• Mitigates proxy-side fallout from DOMAIN_INFOTAINMENT unreachable via signed_command (408 "vehicle is offline") while vehicle is online and DOMAIN_VEHICLE_SECURITY  #450 and Always get the error 408 (vehicle is offline) when send to DOMAIN_INFOTAINMENT by signed_command API #285 for VCSEC-only commands when Infotainment is unreachable
• wake_up skips StartSession entirely over Fleet API

go test ./pkg/proxy/ passes locally.