feat: Add continue_on_error support for workflow steps and actions

[ausias-armesto]

Closes #6553

Description

Adds a continue_on_error attribute to workflow steps and actions. When set to true, a step or action failure is swallowed and execution continues to the next step/action.

Default is false, so existing workflows are unaffected.

Changes

Area	File	What
Backend	keep/step/step.py	continue_on_error attribute + property on Step
Backend	keep/workflowmanager/workflow.py	run_steps() catches StepError and continues; run_actions() suppresses the error, both when the flag is set
Frontend	keep-ui/entities/workflows/model/yaml.schema.ts	continue_on_error: boolean (optional) added to YamlStepOrActionSchema — enables Monaco validation and autocomplete
Tests	tests/test_steps.py	Unit tests for the Step.continue_on_error property
Tests	tests/test_workflowmanager.py	Behavioural tests for run_steps() and run_actions()
Docs	docs/workflows/syntax/steps-and-actions.mdx	New subsection under "Error Handling and Retries"
Docs	docs/workflows/examples/continue-on-error.mdx	New example page with step and action use cases

How to test

steps:
  - name: optional-enrichment
    continue_on_error: true
    provider:
      type: http
      config: "{{ providers.some-api }}"
      with:
        url: "https://api.example.com/enrich/{{ alert.fingerprint }}"

  - name: always-runs
    provider:
      type: slack
      config: "{{ providers.slack-demo }}"
      with:
        message: "Runs even if enrichment failed"

1. Create a workflow with the YAML above.
2. Point optional-enrichment at a URL that returns an error.
3. Verify that always-runs still executes and the workflow execution status is success.
4. Remove continue_on_error: true and verify the workflow now stops at the failing step.