fix(fish): improve shell completion compatibility and namespacing

[SpiliosDimakopoulos]

Description

This PR addresses minor issues in the Fish completion script to ensure better compatibility and avoid potential naming collisions.

Changes

• Namespacing: Renamed __fish_http_verify_options to __fish_httpie_verify_options to prevent conflicts with other HTTP-related plugins.
• Compatibility: Replaced echo -e with printf for better string formatting, as echo -e behavior can vary between shell versions/environments.
• Consistency: Fixed a call-site mismatch where the old function name was still being referenced.

Validation

Verified the syntax using fish --no-execute extras/httpie-completion.fish within a Fish environment. No syntax errors were found.