Make ComponentContext containsKey consistent with get

[sharanggupta]

BaseComponentContext extends LinkedHashMap and overrides get to throw NoSuchElementException when a key maps to a null value, but it inherits the map's containsKey, which reports such a key as present. Guarding get with containsKey, as the API invites, therefore still raised the exception, for example when a ConfirmationInput received input other than y/n and its null result was stored in the context. The maintainer confirmed in the issue that a key reported by containsKey should always be retrievable via get.

This PR overrides containsKey in BaseComponentContext to apply the same rule as get, so a key is only reported as contained when get can return its value, and documents that contract on ComponentContext#containsKey.

Note that current main already null-guards the confirmation result inside ComponentFlow, so the original end-to-end path no longer triggers it, but the inconsistency remains reachable through any put(key, null) (user pre/post handlers, context copying between components).

Tests: two cases added to ComponentContextTests (positive case, and put(key, null) makes containsKey false while get throws). The spring-shell-core and spring-shell-jline suites and spring-javaformat:validate pass.

Fixes #685