observed.composite.resource.spec missing fields on first reconcile causes silent resource drops

## Bug Description

When using `observed.composite.resource.spec` in a Go template, fields that are present in the XR may not be available on the first reconcile cycle. This causes `{{- with .fieldName }}` blocks to silently skip, resulting in composed resources never being created.

## Root Cause

On the first reconcile after XR creation, `observed.composite.resource.spec` may not contain all user-specified fields. The `desired.composite.resource.spec` always has the full spec. When combined with resources that fail quickly (triggering the circuit breaker), the template never gets a second chance to execute with the now-populated observed state.

## Environment

- function-go-templating: v0.11.0
- Crossplane: v1.19.x (latest stable)
- Cluster: Kind (local development)

## Steps to Reproduce

1. Create a Composition with an inline template that uses `{{- with .observed.composite.resource.spec -}}` and accesses multiple sub-objects via `{{- with .service }}`, `{{- with .autoscaling }}`, etc.
2. Include a resource that will fail immediately (e.g., AppAutoscaling Policy that depends on a Target that doesn't exist yet)
3. Create an XR with all fields populated

## Expected Behavior

All `with` blocks should execute on every reconcile since the XR spec is fully populated.

## Actual Behavior

- First reconcile: some `with` blocks don't execute because `observed.composite.resource.spec` is missing those fields
- The failing resource triggers the circuit breaker
- Subsequent reconciles are throttled — the template never re-executes with the complete observed state
- Result: some composed resources are never created

## Evidence

- Decoder test: `yaml.NewYAMLOrJSONDecoder` correctly decodes 6+ documents from well-formed YAML
- `convertToMap` uses `protojson.Marshal(req)` with default options — proto3 omits zero/empty fields
- When the same template uses only 3 resources (no failing ones), all fields are available on the second reconcile (circuit breaker doesn't activate)
- Switching from `observed` to `desired` composite spec resolves the issue completely

## Workaround

Use `desired` with fallback to `observed`:

```go
{{- $spec := (.desired.composite.resource.spec | default .observed.composite.resource.spec) -}}
{{- with $spec -}}
...
{{- end -}}
```

This ensures the full user-specified spec is always available regardless of reconcile timing.

## Recommendation

Consider documenting that `desired.composite.resource.spec` should be preferred over `observed.composite.resource.spec` for resource creation templates, since `desired` always reflects the user's intent while `observed` may lag behind. Alternatively, the function could pre-merge desired and observed composite state before template execution.

Start with the convertToMap path and the yaml.NewYAMLOrJSONDecoder decoder test mentioned in the report, then reproduce a first reconcile with a quickly failing resource and the circuit breaker. Compare observed and desired composite specs during template execution; done means the populated XR fields no longer cause resource-producing with blocks to be skipped, with regression coverage for the first reconcile.

Written by the indexing model from the issue text.