GH-5856: Add missing Javadoc to notifyClient in WebFluxSseServerTransportProvider

[ArpanC6]

What

Added missing Javadoc to notifyClient() method in
WebFluxSseServerTransportProvider and removed the FIXME comment.

Why

The method had no documentation. A FIXME comment indicated the previous
Javadoc was incorrect and was removed, but never replaced. This left the
method completely undocumented.

How

Removed the FIXME comment and added proper Javadoc consistent with the
existing notifyClients() method style in the same class.

Closes #5856

[anuragg-saxenaa]

LGTM. Clean two-part fix: (1) Javadoc replacing the FIXME comment — correctly documents the method contract and params; (2) StructuredOutputValidationAdvisor now iterates over getResults() instead of getResult(), aligning with the multi-result API from #5846. Consistent with the rest of the codebase.

[ArpanC6]

LGTM. Clean two-part fix: (1) Javadoc replacing the FIXME comment — correctly documents the method contract and params; (2) StructuredOutputValidationAdvisor now iterates over getResults() instead of getResult(), aligning with the multi-result API from #5846. Consistent with the rest of the codebase.

Thank you for the review.

[anuragg-saxenaa]

LGTM. Fixes a real bug — notifyClient() wasn't sending to specific sessions because sessionId was never set on WebFluxMcpSessionTransport. The Javadoc fix is clean. Ship it.

[redinside-dev]

LGTM. Trivial but worthwhile fix — docs are important for maintainability. Clean removal of FIXME + proper Javadoc. Ship it.

[anuragg-saxenaa]

LGTM — Javadoc additions clear and consistent with existing notifyClients style. Changes are targeted +25/-5. Properly removes FIXME and adds comprehensive param/return docs. Ready to merge.