test: align extending guide harness with runtime config

[ikenyal]

Problem

The BlocksBackend guide harness assumes the generated template contains exactly one Lambda. The config registry also creates Lambda-backed CDK custom resources, so the current assertion fails even when the Blocks backend is correct. The guide harnesses also use handler.addEnvironment() for resource identifiers, contrary to the current registerConfig() guidance.

Issue #, if available: N/A

Changes

• Register the guide fixture resource identifiers through registerConfig().
• Verify resource identifiers are included in the generated runtime config rather than direct Lambda environment variables.
• Identify the Blocks handler by its production runtime environment instead of counting all Lambda resources.

Validation

• Ran npm run build.
• Ran npm run lint:deps.
• Ran npx tsc --noEmit -p test-apps/extending-blocks-guide/tsconfig.json.
• Ran npx tsc --noEmit -p test-apps/extending-blocks-guide-blocksbackend/tsconfig.json.
• Ran npm run test:mock --workspace test-extending-blocks-guide.
• Ran npm run test:synth --workspace test-extending-blocks-guide.
• Ran npm run test:synth --workspace test-extending-blocks-guide-blocksbackend.

Checklist

• PR description included
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

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