Reject empty env var keys

[CodingFeng101]

fastmcp run --env and the install helpers both accept environment variables as KEY=VALUE. Today a value like =secret or =secret passes validation and produces an empty environment variable name after stripping. That can write invalid server configuration or fail later in a less direct place.

This rejects empty keys at the parse boundary while preserving the existing value parsing behavior, including empty values such as EMPTY_VAR= and values containing additional = characters.

_parse_env_var("  =secret")  # exits with a clear validation error
_parse_env_var("EMPTY_VAR=")  # still accepted