Gateway commands: UX improvements, refactoring, and test coverage

[leggetter]

Summary

Follow-up items from the code review of #213 that were not addressed in #219. These are UX enhancements, refactoring, and test coverage improvements — no bugs.

---

1. Delete/enable/disable commands only accept IDs, not names

Priority: Low
Files: source_delete.go, source_enable.go, destination_delete.go, destination_enable.go, etc.

get commands support <id-or-name> via the resolve*ID functions, but delete, enable, and disable only accept bare IDs. Users can get my-source but must delete src_abc123.

---

2. Significant boilerplate across resource types

Priority: Medium
Files: source_list.go, destination_list.go, event_list.go, enable/disable commands, etc.

List commands follow an identical pattern: build params → call client → branch on --output json → format text. Enable/disable commands are nearly identical ~45-line files with only the resource type swapped.

A factory function like makeEnableCmd(resource, clientFn) would eliminate ~200 lines of duplication.

---

3. Hardcoded magic strings in source_common.go

Priority: Medium
Files: source_common.go:62-64,76,81

Default auth config values like "sha256", "hex", "x-webhook-signature", "x-api-key" are duplicated across buildSourceConfigFromIndividualFlags and normalizeSourceConfigAuth.

Extract to named constants.

---

4. event_list has 25+ filter flags mapped manually

Priority: Low
Files: event_list.go:100-172

~70 lines of if field != "" { params[key] = field }. A helper like addNonEmpty(params, "key", value) or struct tags would be cleaner and less error-prone.

---

5. --connection-id maps to API param webhook_id

Priority: Low
Files: event_list.go:104

params["webhook_id"] = ec.connectionID

The mismatch between the user-facing flag name and the API parameter name is confusing. At minimum add a comment; ideally define a constant APIParamConnectionID = "webhook_id".

---

6. No tests for new resource commands

Priority: Medium

The PR added ~50 new command files but only 2 new test files. Destination, transformation, event, request, and attempt commands have no unit tests.

---

Remaining items from the code review of #213. See #217 for the original issue; bugs were fixed in #219.