chore: generate examples SAM template

[zhongkechen]

Summary

• Replace the checked-in examples/template.yaml with examples/generate-template.py so the SAM template is generated from deployable Java example handlers.
• Add source-retention @ExampleTemplate metadata for explicit template-only behavior such as Java-version conditions and OTel/X-Ray tracing, avoiding path-based special cases in the generator.
• Generate CloudWatch log groups for every example Lambda function with RetentionInDays: 7.
• Add a one-time unmanaged log group cleanup helper and run it before e2e sam deploy so existing Lambda-created log groups do not block CloudFormation adoption.
• Update the e2e workflow, local build helper, README, and .gitignore for the generated template flow.

Details

• The generator scans top-level Java classes extending DurableHandler under examples/src/main/java/software/amazon/lambda/durable/examples.
• Function names are derived from class names using kebab-case and retain the existing FunctionNamePrefix behavior.
• @ExampleTemplate(condition = "IsJava21OrLater") is used for the virtual-thread example.
• @ExampleTemplate(tracing = true) is used for OTel X-Ray examples to emit Tracing: Active and the ADOT Lambda layer.
• examples/template.yaml is now ignored and generated on demand before sam build/deploy.
• The cleanup script is dry-run by default outside CI; CI passes --execute for the known e2e stacks. The workflow includes a TODO noting that this migration cleanup can be removed later.

Verification

• python3 examples/generate-template.py --output /tmp/generated-template.yaml
• custom functional comparison: generated template preserves all 40 existing Lambda functions and 40 outputs, and adds one 7-day log group per function
• HOME=/tmp SAM_CLI_TELEMETRY=0 sam validate --lint --template-file /tmp/generated-template.yaml
• python3 -m py_compile examples/generate-template.py
• bash -n .github/scripts/cleanup_e2e_unmanaged_log_groups.sh
• bash -n examples/build-and-run.sh
• mvn spotless:apply
• mvn -pl examples -am -DskipTests compile
• git diff --check