cmd: subscribe silently ignores malformed --filter values

## Problem

`func subscribe --filter` expects filters in `key=value` form, but malformed values are currently printed and ignored instead of returning an error.

For example:

```bash
func subscribe --filter badfilter
```

prints `Invalid pair: badfilter`, continues, and can still write subscription config without the intended filter.

## Expected behavior

Malformed filters should fail before `func.yaml` is changed. Valid values containing `=` should still be preserved, for example `--filter foo=bar=baz` should store key `foo` with value `bar=baz`.

## Suggested fix

Return an error for missing `=` or an empty key, and add regression tests for malformed filters and values containing `=`.