fix: Exact optional property types

[JeongJuhyeon]

What kind of change does this PR introduce?

Bug fix and TS config update

What is the current behavior?

The StorageFileApi.bucketId field type is bucketId?: string, which breaks when exactOptionalPropertyTypes is on. This causes headaches for folks trying to use the library with the flag on.

What is the new behavior?

1. Fixed StorageFileApi.bucketId field type: bucketId: string | undefined
2. Enabled "exactOptionalPropertyTypes": true in TS config

Now it should play nice with the flag on without breaking existing setups.

Additional context

Here's why I think this change is useful:

• TypeScript itself recommends using this flag. The reason it is not in strict is just to avoid breaking too much existing user code.

• Right now, anyone with this flag on can't use the library without it failing to compile. Using skipLibCheck isn't great because it skips all .d.ts files, including user-made ones (see this issue).

• Except for the one field I fixed, the package already follows this rule.

• In my current project with 58 libraries, this is the only one that breaks with the flag on, indicating it's already a standard in the ecosystem.

These tweaks should make the library more robust and play better with strict TS setups, without causing issues for current users. Let me know if you need any more info!