fix(s3): make Bucket assignable to IBucket under exactOptionalPropertyTypes

[laazyj]

Issue # (if applicable)

Refs #37996 (reported by @mingkunm on the issue).

Reason for this change

IBucket.isWebsite is declared optional (?: boolean), but Bucket exposed it through a getter typed boolean | undefined (delegating to BucketReflection). Under TypeScript's exactOptionalPropertyTypes, a member typed T | undefined is not assignable to an optional ?: T, so consumers who enable that flag hit a TS2375 error when passing a Bucket where an IBucket is expected (e.g. BucketProps.serverAccessLogsBucket), and type-checking aws-cdk-lib fails for them outright:

const b: IBucket = new Bucket(stack, 'B'); // TS2375 under exactOptionalPropertyTypes

aws-cdk-lib does not build with the flag, so the break is invisible to the library's own compile. Notably, Bucket satisfies IBucket through BucketBase (which declares abstract readonly isWebsite?: boolean), so there is no TS2420 at the class — the break only surfaces as a call-site assignment failure. This is one package in the sweep tracked by #37996.

Description of changes

Convert isWebsite (on Bucket and the internal fromCfnBucket subclass) from a getter to a public readonly isWebsite?: boolean field, set in the constructor from this.reflection.isWebsite once the underlying CfnBucket (with its website configuration) exists.

• Narrowing the getter to : boolean is not an option — jsii rejects a required getter overriding the optional abstract member (JSII5009). The readonly field is the shape that satisfies both jsii and the flag.
• The property stays optional and immutable in the jsii assembly, so this is not an API change; yarn compat passes with no new entry.
• Website hosting on an L2 Bucket is configured by props at construction (there is no post-construction L2 API to enable it), so capturing the value at construction is behaviour-preserving for supported usage. The existing isWebsite unit tests are unchanged and pass.

New: an auto-enumerating regression guard

This PR introduces a shared helper in @aws-cdk/cdk-build-tools, exactOptionalAssignabilityDiagnosticsForPackage(testDir), which enumerates every exported, non-abstract class in a package and type-checks its assignability to each interface it implements (its own and inherited) under the flag. Enumerating — rather than hand-listing classes — keeps the guard complete by construction: a newly-added offending class (or a new optional getter on an existing one) is caught automatically. The aws-s3 guard test is a one-liner over this helper, and subsequent sweep packages can adopt it the same way.

Description of how you validated your changes

• The package-local guard (aws-s3/test/exact-optional-property-types.test.ts) passes; verified it goes red with the precise diagnostic Bucket -> IBucket: TS2375 [isWebsite] if the fix is reverted.
• The existing isWebsite behaviour tests (default false, true for each website configuration) are unchanged and pass; full aws-s3 suite green.
• yarn compat passes (no assembly change); lint clean.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

[laazyj]

Exemption Request: This is a type-only change — it converts an existing read-only getter (Bucket.isWebsite) to an equivalent readonly field so the class is assignable to its interface under exactOptionalPropertyTypes. There is no runtime behaviour change and no change to synthesized output or the jsii assembly (the property stays optional and immutable; yarn compat passes with no new entry), so an integration test would add nothing. The change is covered by the existing isWebsite unit tests plus a package-local type-assignability guard. Requesting the pr-linter/exempt-integ-test label.

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

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