refactor(config): separate source loading from resolution

[natalie-o-perret]

Description

Follow-up to #817.

Splits config source loading from resolution into a small, testable layer. Each source (env vars, config file profile, built-in defaults) is loaded independently, then merged by resolve() with explicit precedence:

CLI flags > env vars > config file profile > built-in defaults

What changed

• New cmd/config_resolution.go with envSources, fileSources, and resolve(). Replaces the old envAccountOverrides / useEnvOnlyAccount / finalizeCurrentAccount tangle.
• Fixed --config vs EXOSCALE_CONFIG precedence: the env-to-flag mapping was missing a Changed guard, so EXOSCALE_CONFIG silently won. CLI flags now always win.
• Fixed EXOSCALE_ZONE: it now overrides the default zone from the active config profile as documented.
• Fixed a race where Ctrl+C during the secret key prompt killed the process before it could print the cancellation message (three timing paths for SIGINT covered, plus SIGHUP from PTY session leader exit).
• 11 unit tests for resolve(), 2 new e2e scenarios for the fixed behaviors.
• With-API e2e CI split into parallel compute/dbaas jobs for independent failure reporting.

Examples

2 config files used in the demos:

flag-config.toml:

defaultaccount = "flag-account"
[[accounts]]
name = "flag-account"
key = "EXOflag000000000000"
secret = "flagsecret..."
defaultZone = "ch-gva-2"

env-config.toml:

defaultaccount = "env-account"
[[accounts]]
name = "env-account"
key = "EXOenv0000000000000"
secret = "envsecret..."
defaultZone = "de-fra-1"

zone-config.toml:

defaultaccount = "zone-account"
[[accounts]]
name = "zone-account"
key = "EXOzone000000000000"
secret = "zonesecret..."
defaultZone = "de-fra-1"

---

Bug 1: --config flag ignored when EXOSCALE_CONFIG is set

EXOSCALE_CONFIG=env-config.toml exo --config flag-config.toml config show

Before (master): EXOSCALE_CONFIG silently overrides the flag:

│ Name               │ env-account                   │
│ Default Zone       │ de-fra-1                      │
│ Configuration File │ /tmp/exo-demo/env-config.toml │

After (this PR): --config flag wins as expected:

│ Name               │ flag-account                   │
│ Default Zone       │ ch-gva-2                       │
│ Configuration File │ /tmp/exo-demo/flag-config.toml │

---

Bug 2: EXOSCALE_ZONE has no effect when a config file is present

EXOSCALE_ZONE=ch-gva-2 EXOSCALE_CONFIG=zone-config.toml exo config show
# zone-config.toml has defaultZone = "de-fra-1"

Before (master): env var ignored, config zone used:

│ Name               │ zone-account                   │
│ Default Zone       │ de-fra-1                       │
│ Configuration File │ /tmp/exo-demo/zone-config.toml │

After (this PR): EXOSCALE_ZONE overrides as expected:

│ Name               │ zone-account                   │
│ Default Zone       │ ch-gva-2                       │
│ Configuration File │ /tmp/exo-demo/zone-config.toml │

Testing

go test ./cmd/...
make build && cd tests/e2e && go test -run TestScriptsLocal -count=1 .

Note

GitHub Copilot leveraged to:

• Generate boilerplate testing code and assertions in the configuration resolution logic
• Rephrase this very PR description and comments in the implementation

[natalie-o-perret]

[SC-178713]