feat(cli-internal): add lambda layer support to gen2-migration generate command

[adrianjoshua-strutt]

Description of changes

The gen2-migration generate command now handles Lambda layers
attached to functions. Previously, layer configurations were silently
dropped during migration — users had to manually recreate them in
Gen2 after running the tool.

The generate command already fetches FunctionConfiguration from the
AWS SDK (which includes Layers), but the field was never read. This
PR reads the layer data from the deployed function config and
generates Gen2's native defineFunction({ layers }) property.

Key design decision: The AWS SDK returns layer ARNs but not the
module names that layers export. Gen2's layers record uses module
names as keys (for bundler externalization). Since the actual module
name isn't available from the API, we extract the layer name from the
ARN and use it as the key. Users may need to update the key to match
their actual import path after migration.

Core logic

FunctionGenerator.resolve() now calls extractLayers() to parse
the config.Layers array into a Record<string, string>. The layer
name is extracted from the ARN (segment 6 in the colon-delimited
format). FunctionRenderer.renderLayers() emits the record as an
AST object literal in the defineFunction() call, following the same
pattern used for environment.

extractLayers() validates ARN structure (minimum 8 segments,
resource type must be layer) and throws on duplicate layer names
with a descriptive error message.

Test fixtures

Updated the project-boards test app to include two Lambda layers
on the quotegenerator function. Updated the LambdaMock to read
Layers from CFN templates and return them in the
GetFunctionCommand response.

Documentation

Updated GEN2_MIGRATION_GUIDE.md to change Lambda layer support
from 🔴 (unsupported) to 🟡 (generate ✔, refactor ✗).

Issue #, if available

Closes #14527

Description of how you validated changes

• All 12 snapshot tests pass (9 apps + 3 unit), including the
  updated project-boards snapshot with layers in the expected
  defineFunction() output
• 9 new unit tests for extractLayers() covering: undefined input,
  empty array, missing ARNs, malformed ARNs, non-layer ARNs,
  duplicate layer names, 5-layer maximum, and the happy path
• TypeScript compilation clean (no new errors)
• All pre-commit hooks pass (sanitize, prettier, lint, commitlint,
  git-secrets)

Checklist

• PR description included
• yarn test passes
• 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

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.