fix(config): improve config override behavior

[nhedger]

Note

Disclosure:

• AI assistance was used to implement this feature
• Manual review and testing was done

Description

Fixes #737

This updates config resolution so environment variables override account credentials without discarding the rest of the selected profile.

Previously, when EXOSCALE_API_KEY and EXOSCALE_API_SECRET were set, the CLI returned early during config initialization and skipped loading the config file entirely. As a result, profile values such as defaultZone were ignored and commands relying on the account default zone could fall back to the built-in default instead of the zone configured in exoscale.toml.

This change now:

• loads and selects the config profile first when a config file is available
• applies environment overrides afterwards for credentials and related override fields
• preserves profile settings such as defaultZone and other account defaults
• keeps the existing env-only behavior when no config file is present
  This fixes the override precedence so env credentials replace authentication settings, not the whole profile.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block, and add the Pull Request #number for each bit you add to the CHANGELOG.md)
• Testing

Testing

• Ran go test ./...
• Ran targeted e2e coverage for the config/env precedence:
  • go test -run 'TestScriptsLocal/(show_with_env_credentials_keeps_config_defaults|show_with_env_credentials_without_config)$' -count=1 ./... from tests/e2e
• Verified that:
  • env credentials still work without a config file
  • defaultZone from the config is preserved when env credentials are set
• Confirmed that [Bug]: Default zone is not respected #737 cannot be reproduced anymore:

  export EXOSCALE_API_KEY="..."
  export EXOSCALE_API_SECRET="..."
  VOL_ID=$(./bin/exo --output-format json compute block-storage create repro-737-$(date +%s) --zone ch-gva-2 --size 10 | jq -r '.id')
  ./bin/exo compute block-storage list
  ./bin/exo compute block-storage show "$VOL_ID"
  ./bin/exo compute block-storage show "$VOL_ID" --zone ch-gva-2
  ./bin/exo compute block-storage delete "$VOL_ID" --zone ch-gva-2 -f