sec(server): validate environment key names for control characters in exec requests

[cluster2600]

Summary

In `crates/openshell-server/src/grpc.rs` (line 3452), the `validate_exec_request_fields()` function applies `reject_control_chars()` to environment values but not to environment keys. Whilst keys come from the gRPC map (which limits exploitability), control characters in key names could cause unexpected behaviour in shell environments.

Impact

• Severity: Low

Proposed Fix

Apply `reject_control_chars` to environment key names as well as values.

[johntmyers]

Security triage determination: No-go (low-value input hardening, no boundary escalation).

Applying control-character validation to env keys is defensible hygiene, but callers of this path already require high privilege (exec capability), and this ticket does not demonstrate new privilege gain or cross-boundary compromise.

Keep as low-priority robustness cleanup if desired, not a security fix candidate.