fix(amplify-velocity-template): block prototype-chain property access in vtl reference resolver

[sarayev]

Description of changes

The VTL reference resolver mapped property, index, and method access in a
template directly onto JavaScript property lookups with no restriction on
which names could be resolved. Because JavaScript objects expose their
internal machinery through the prototype chain, a template could walk from
any context value up to the Function constructor — for example
$ctx.constructor.constructor("...") — and from there build and invoke
arbitrary code, escaping the intended template sandbox.

This change hardens the resolver so that JavaScript-internal, prototype-chain
property names can never be resolved or invoked from a template.

Blocked-property guard in the reference resolver

Adds a single isBlockedProperty guard backed by a small list of
prototype-chain names: constructor, __proto__, prototype,
__defineGetter__, __defineSetter__, __lookupGetter__, and
__lookupSetter__. The guard is applied consistently to all three access
paths the resolver supports:

• dot / property access ($ctx.constructor)
• bracket / index access ($ctx['constructor'])
• method-call access ($ctx.constructor(...))

When a template references one of these names it now resolves to an undefined
reference, so it renders as empty (or the literal text under non-silent
reference notation) and can never be called. This closes the path from a
context value to the Function constructor.

Backward compatibility

Only these JavaScript-internal names change behavior. Normal data access and
method calls are unaffected — $ctx.name, $ctx['name'],
$str.toUpperCase(), $list.size(), and $map.keySet() all continue to
work exactly as before. Real template data has no legitimate need to resolve
prototype-chain properties, so this narrows the resolver's surface without
affecting valid templates.

Issue #, if available

N/A

Description of how you validated changes

Added regression tests in packages/amplify-velocity-template/tests/references.test.js:

• Negative assertions that dot, bracket, and chained constructor /
  __proto__ / prototype access all render empty.
• An execution-safety assertion that a chained constructor invocation
  ($!ctx.constructor.constructor("globalThis.__vtlMarker = true").call())
  renders empty and leaves the global marker unset, proving no code was
  executed.
• Positive regressions confirming normal property access and normal method
  calls (toUpperCase(), size(), keySet()) still resolve correctly.

Ran the full package suite with mocha:

153 passing (59ms)

0 failing.

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.