feat(gen2-migration): generate code for non-JS Lambda functions

[sharonyajain]

Description of changes

Resolves #14535 — gen2-migration generate now produces Gen2 code for non-JS Lambda functions (Python, Go, Java, .NET, Ruby) instead of throwing or skipping them.

Previously, the generate command only supported Node.js runtimes. PR #14744 added a skip path (via --skip-validations), but left users to hand-code the Gen2 definition. This PR completes the feature by generating the full resource.ts using Gen2's custom functions pattern.

What it generates (example for Python):

import { execSync } from 'node:child_process';
import * as path from 'node:path';
import { fileURLToPath } from 'node:url';
import { defineFunction } from '@aws-amplify/backend';
import { DockerImage, Duration } from 'aws-cdk-lib';
import { Code, Function, Runtime } from 'aws-cdk-lib/aws-lambda';

const functionDir = path.dirname(fileURLToPath(import.meta.url));

export const myPythonFunc = defineFunction(
  (scope) =>
    new Function(scope, 'myPythonFunc', {
      handler: 'index.handler',
      runtime: Runtime.PYTHON_3_11,
      timeout: Duration.seconds(20),
      memorySize: 256,
      code: Code.fromAsset(functionDir, {
        bundling: {
          image: DockerImage.fromRegistry('dummy'),
          local: {
            tryBundle(outputDir: string) {
              execSync(`python3 -m pip install ...`);
              execSync(`cp -r ...`);
              return true;
            },
          },
        },
      }),
    }),
);

Key changes:

• FunctionAssessor: All runtimes are now supported for generate (removed the isNonJsRuntime gate)
• FunctionRenderer: New renderCustomFunction() method emits the CDK Function construct pattern with runtime-specific bundling
• FunctionGenerator: Removed the throw for non-JS runtimes; branches into custom path when !runtime.startsWith('nodejs')
• copyFunctionSource: Preserves dependency files (requirements.txt, go.mod, pom.xml) for non-JS functions
• mapToCdkRuntime(): Maps Lambda runtime strings → CDK Runtime.* enum members

Runtime support:

Lambda Runtime	CDK Enum	Bundling
python3.x	PYTHON_3_X	pip install + copy
go1.x / provided.al2023	PROVIDED_AL2023	go build
java*	JAVA_*	mvn package
dotnet*	DOTNET_*	dotnet publish
ruby*	RUBY_*	copy

Issue #, if available

Closes #14535

Description of how you validated changes

• Runtime mapping unit tests (7 runtimes → correct CDK enum)
• Custom function rendering tests for Python, Go, Java, .NET
• Assessor tests updated: all runtimes pass validFor('generate')
• Manual verification of generated output matches Gen2 docs
• Pre-existing test for does not render runtime for non-nodejs still passes (Node.js path unchanged)

Checklist

• PR description included
• yarn test passes (gen2-migration tests)
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

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