[fix] never send null template variables to Aliyun SMS

[orangeCatDeveloper]

Fixes #4319

Sending a test SMS through Alibaba Cloud always fails with Aliyun's "模板变量JSON格式错误 或 JSON变量属性与模板占位符不一致" error, regardless of configuration — and the docs make it worse by showing a template example in the wrong placeholder syntax.

Root cause: the hardcoded test alert built by sendTestMsg carries no instance label and no groupKey, while AlibabaSmsClientImpl assumes real-alert shape and falls through to templateParam.put("instance", null). Aliyun rejects any request whose template-variable JSON contains a null/blank value, so the test path was broken for every user.

Three layers:

• AlibabaSmsClientImpl: param building extracted into buildTemplateParam, every variable falls back to non-blank text (instance → groupKey → "unknown"; content → summary → description → any annotation → "alert triggered"). Null can no longer reach the JSON, for test and real alerts alike; also removes an NPE when commonAnnotations is null.
• sendTestMsg: the shared test alert now carries an instance label — all five SMS providers read it.
• Docs (en/zh): the Aliyun template example used backticks; corrected to Aliyun's ${...} syntax with a note that the template must declare exactly these three variables.

Boundary: per-rule SMS templates (question 1 in the issue) is a feature request, not covered here — suggested as a separate issue in the discussion. No Aliyun account was available for a live send (custom templates require review), so evidence stops at the outbound JSON — which is what Aliyun validates.

Before / after (the exact TemplateParam sent to Aliyun for the test alert):

before: {"instance":null,"priority":"unknown","content":"Please check the CPU usage of the server"}
        -> rejected: template variable JSON error / placeholder mismatch (the issue error)

after:  {"instance":"127.0.0.1","priority":"unknown","content":"Please check the CPU usage of the server"}

Verified: 3 new unit tests (no-null for test-alert shape, no-null for empty alert, real values pass through untouched); full alerter suite 468/468; end-to-end smoke of the shared send-test-msg path via the email channel against a local SMTP server (message rendered and delivered).