feat(basic-auth): add context key and callback options (#4645)

* feat(basic-auth): add usernameContextKey and onAuthSuccess options

Add two new options to basicAuth middleware:
- `usernameContextKey`: stores authenticated username in context
  - `true`: uses default key 'basicAuthUsername'
  - `string`: uses custom key name
- `onAuthSuccess`: callback invoked after successful authentication

This allows route handlers to access the authenticated username without
re-parsing the Authorization header.

* test(basic-auth): add tests for usernameContextKey and onAuthSuccess

Add test cases for:
- usernameContextKey with default key 'basicAuthUsername'
- usernameContextKey with custom key
- usernameContextKey not set (should not store username)
- usernameContextKey with verifyUser mode
- onAuthSuccess callback execution
- onAuthSuccess async callback support
- onAuthSuccess not called on failed auth
- onAuthSuccess with verifyUser mode
- Both options used together

* remove `usernameContextKey`

* remove unnecessary comments

* remove unnecessary comments

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Co-authored-by: Yusuke Wada <yusuke@kamawada.com>