Fix region command whitespace parsing

[axhoff]

Summary

• ignore empty fields caused by repeated or trailing spaces in region commands
• preserve the existing parser API and behavior for every other command
• add regression coverage for trailing spaces, repeated spaces around a region name, and explicit <null>

Root cause

The generic text splitter preserves empty fields. For region default , that produced an empty third argument. findByNamePrefix("") then matched the last region in the table, after which the default-region handler also cleared its deny-flood flag and persisted the change.

Region commands now opt into a separately named splitter that ignores empty separator fields. Bare region default, including trailing spaces, remains the documented read-only query. Clearing the default remains explicit: region default <null>.

The original four-argument Utils::parseTextParts symbol and behavior are unchanged.

Impact

Trailing or repeated spaces can no longer select a region accidentally, change its flood policy, or persist an unintended default scope.

Fixes #2992

Validation

• native test binary: 9/9 tests passed
• pio test -e native
• pio run -e RAK_4631_repeater
• pio run -e Station_G2_repeater
• git diff --check
• adversarial review: final ship recommendation after preserving the original parser ABI