PMM-15145 Fix DB down alerts auto-resolving when agent is down

[theTibi]

Problem

MongoDB/PostgreSQL/Valkey/Redis down alerts share the flaw fixed for MySQL in
PMM-14193: when the DB and its agent/exporter go down together, the *_up
metric disappears, the expression has no series to evaluate, and the alert
silently resolves — exactly when it should fire.

Fix

Anchor on pmm_managed_inventory_agents (emitted by pmm-managed, always present).
Any enabled service not currently reporting *_up == 1 (down or missing)
stays at value 1 → Alerting; a baseline 0 keeps healthy ones Normal. Keying on
service_id also fixes the old on(node_name) join for multi-instance nodes.

Updated: mongodb_down, postgresql_down, valkey_down, redis_down — same
expression as PMM-14193, only metric/agent_type/labels differ.

Not changed — agent_down: its source pmm_managed_inventory_agents{agent_type="pmm-agent"}
flips to 0 (doesn't vanish) when the host dies, so it has no missing-series issue.

Note: redis_down and valkey_down are identical (both redis_up/valkey_exporter,
no service_type label to split them) — pre-existing, preserved here.

Ref: PMM-14193

Ticket number: PMM-0

Feature build: SUBMODULES-0

If this PR adds, removes or alters one or more API endpoints, please review and update the relevant API documentation as well:

• API Docs updated

If this PR is related to other PRs, contributions, or ongoing work in this or other repositories, please reference them here:

• Links to related work items (optional).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.54%. Comparing base (0e531ee) to head (a8e5112).

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #5492      +/-   ##
==========================================
- Coverage   43.62%   43.54%   -0.08%     
==========================================
  Files         413      413              
  Lines       42401    42928     +527     
==========================================
+ Hits        18498    18694     +196     
- Misses      22144    22362     +218     
- Partials     1759     1872     +113     

Flag	Coverage Δ
managed	42.86% <ø> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.