fix(grafana): always set optional alert fields to avoid render_context errors

[hakatt]

Summary

Fixes #5070 and the general class of "Could not find key 'alert.X'" errors that occur when a workflow template references an alert field that is absent from the context.

Root cause

render_context() in iohandler.py renders all provider with: parameters using Chevron/Mustache with safe=True. When Chevron encounters {{ alert.someField }} and that field doesn't exist in the alert context dict, it writes "Could not find key alert.someField" to stderr — which _render() detects and raises a RenderException.

This affects schema fields and extra fields differently:

• Schema fields on AlertDto (e.g. url, description) always exist in the context because they have None defaults — Chevron renders None as "", no error.
• Extra fields (via extra = Extra.allow) that are never set are genuinely absent from the context dict, which triggers the error.

For the Grafana provider, several useful fields (panelUrl, dashboardUrl, silenceURL, valueString, value, datasource) fall into this second category — they're only populated for certain alert types (webhook vs. datasource alerts) and were previously set to None or not set at all when absent.

Fix

Always set these optional extra fields as "" when absent, in both the webhook alert path (_format_alert) and the datasource alert path (_get_alerts_datasource). Workflow templates can then safely reference them and use or-chains for fallbacks:

# This now works even when panelUrl is absent
'link': '{{ alert.panelUrl }}' or '{{ alert.dashboardUrl }}' or 'N/A'

The same pattern should be applied to other providers where fields are conditionally present. The underlying issue is that render_context() uses safe=True uniformly — a more general fix could be to allow providers or individual parameters to opt out of safe rendering, but that is a larger change.

Test plan

• Grafana webhook alert (has panelUrl, dashboardUrl, silenceURL, valueString) — optional fields populated correctly
• Grafana datasource alert (has datasource, value) — webhook-only fields default to ""
• Workflow template referencing absent Grafana fields no longer raises RenderException
• Existing Grafana provider tests pass

🤖 Generated with Claude Code

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[hakatt]

Blocked by #5676 — the frontend-tests check is failing on all PRs due to an out-of-sync package-lock.json (missing @types/react@18.3.28). Once that merges, CI should go green here too.

[shahargl]

lgtm

[shahargl]

@hakatt can you fix "test docs" so it will pass?