…support

This commit addresses issue #19018 by adding two key features to the Coder Helm chart:

1. **Pod-level securityContext Support**
   - Added coder.podSecurityContext configuration option
   - Enables setting fsGroup for proper file permissions when mounting TLS certificates
   - Supports all standard Kubernetes pod security context fields
   - Example: fsGroup: 1000 for coder user certificate access

2. **Enhanced Probe Configuration**
   - Extended readiness and liveness probe configuration beyond initialDelaySeconds
   - Added support for: periodSeconds, timeoutSeconds, successThreshold, failureThreshold
   - Maintains backward compatibility - new fields only included when explicitly set
   - Enables fine-tuning probe behavior for production deployments

**Use Case:**
This enables secure mTLS database connections by allowing proper certificate
mounting with correct file permissions, addressing enterprise deployment
requirements for PostgreSQL with SSL/mTLS.

**Example Configuration:**
```yaml
coder:
  podSecurityContext:
    fsGroup: 1000
    runAsNonRoot: true
  readinessProbe:
    periodSeconds: 10
    timeoutSeconds: 5
    failureThreshold: 3
  livenessProbe:
    periodSeconds: 30
    timeoutSeconds: 10
    failureThreshold: 3
```

**Testing:**
- All existing Helm chart tests pass
- Template rendering validated with new configurations
- Backward compatibility maintained

Fixes #19018

Co-authored-by: bpmct <22407953+bpmct@users.noreply.github.com>

…mpatibility

The provisioner chart uses the same libcoder templates but doesn't have
probe configuration in its values.yaml. This change makes the probe
configuration conditional to prevent nil pointer errors when the
provisioner chart is rendered.

Changes:
- Wrap readinessProbe and livenessProbe blocks with conditional checks
- Only render probe configuration when .Values.coder.readinessProbe/.livenessProbe exist
- Maintains backward compatibility for both coder and provisioner charts

Fixes helm lint failures in CI.