[BUG] CLI fails with misleading error when AsyncAPI YAML contains multiple documents

[sahillllllllll-bit]

Describe the bug.

When an AsyncAPI file contains multiple YAML documents (separated by ---), the AsyncAPI CLI fails with a low-level and misleading parser error instead of explicitly detecting and rejecting the condition.

While AsyncAPI itself supports only a single document, multi-document YAML is valid YAML and commonly appears in real-world workflows. The CLI should therefore fail explicitly and clearly, rather than falling into an incorrect parsing code path.

---

Why multiple YAML documents appear in practice

Multi-document YAML is widely used and often introduced unintentionally:

• Common in Kubernetes and DevOps configurations
• Users may merge or concatenate YAML files
• Tooling or generators may emit multiple documents by default
• Copy–paste workflows may leave --- separators behind
• Users may comment out one document but leave the separator

Even if AsyncAPI does not support multi-document files, users reasonably expect the CLI to detect and clearly reject this case.

Expected behavior

When an AsyncAPI file contains multiple YAML documents, the CLI should:

• Detect that multiple YAML documents are present
• Fail early during validation
• Return a clear, user-facing error explaining that only a single AsyncAPI document is supported

Example expected output:

Error: AsyncAPI files with multiple YAML documents are not supported.

Screenshots

How to Reproduce

1. Create an AsyncAPI file containing multiple YAML documents:

asyncapi: '2.6.0'
info:
  title: Test API
  version: '1.0.0'
channels: {}

---
asyncapi: '2.6.0'
info:
  title: Second Doc
  version: '1.0.0'
channels: {}

2. Run the AsyncAPI CLI:

asyncapi generate fromTemplate asyncapi.yaml ./test-template --no-interactive

3. Observe the CLI output.

🖥️ Device Information [optional]

• Operating System (OS):Windows(WSL)
• Browser:
• Browser Version:

👀 Have you checked for similar open issues?

• I checked and didn't find similar issue

🏢 Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

Are you willing to work on this issue ?

Yes I am willing to submit a PR!

[sahillllllllll-bit]

@Shurtu-gal @AayushSaini101 Waiting for the approval to submit PR thanks :)