feat(test): document required positional args and use cobra.ExactArgs

[hazelmayank]

Summary

Make microcks test --help self-documenting and replace the manual
arg-count check with Cobra's declarative Args: cobra.ExactArgs(3).

Fixes #446 .

Background

microcks test --help previously showed:

Usage:
  microcks test [flags]

…with no indication that three positional arguments are required. The contract
was only discoverable by running the command with no args and reading the
error printed by a manual if len(os.Args) < 4 check in the Run body.

Changes

cmd/test.go:

1. Document the positional args in Cobra metadata.

   • Use: is now test <apiName:apiVersion> <testEndpoint> <runner>.
   • Long: describes each required argument and the runner choices.
   • New Example: block shows a realistic invocation including the
     Keycloak service-account flags used in CI.

2. Use Cobra's declarative arg validation.

   • Args: cobra.ExactArgs(3) is set on the command. Cobra now rejects
     wrong-arg-count invocations with a usage hint, before the Run
     body even fires.
   • Removed the now-redundant if len(os.Args) < 4 { ... os.Exit(1) }
     block in Run.

The per-argument empty-string and dash-prefix checks are intentionally kept
as defense-in-depth.

No change to flags, server interaction, exit codes for the valid path,
or the runner-validation behavior.

Before / after

Before:

Usage:
  microcks test [flags]

After:

Usage:
  microcks test <apiName:apiVersion> <testEndpoint> <runner> [flags]

Required arguments:
  <apiName:apiVersion>  Service reference, e.g. "Beer Catalog API:0.9".
  <testEndpoint>        URL of the deployed implementation under test.
  <runner>              Test strategy. One of: HTTP, SOAP_HTTP, SOAP_UI,
                        POSTMAN, OPEN_API_SCHEMA, ASYNC_API_SCHEMA,
                        GRPC_PROTOBUF, GRAPHQL_SCHEMA.

Examples:
  microcks test "Beer Catalog API:0.9" http://my-service/api/ POSTMAN \
    --microcksURL=http://microcks.example.com/api \
    --keycloakClientId=microcks-serviceaccount \
    --keycloakClientSecret=<secret> \
    --waitFor=10sec

Test plan

• go build . passes.
• microcks test --help shows the new docs.
• microcks test (no args) prints Cobra's "accepts 3 arg(s), received 0"
  with the new Usage hint, and exits non-zero.
• microcks test a b c d (4 args) is rejected with "accepts 3 arg(s),
  received 4".
• CI passes on all platforms.