Closes gh-27216

This commit improves the documentation for @Cacheable to point out that
`null` will be stored in the cache for an empty `Optional` return value.

Closes gh-27184

Prior to this commit, if the TaskExecutor configured in
WebSocketMessageBrokerStats for the inboundChannelExecutor or
outboundChannelExecutor was not a ThreadPoolTaskExecutor, a
StringIndexOutOfBoundsException was thrown when attempting to parse the
results of invoking toString() on the executor.

The reason is that ThreadPoolTaskExecutor delegates to a
ThreadPoolExecutor whose toString() implementation generates text
containing "pool size = ...", and WebSocketMessageBrokerStats'
getExecutorStatsInfo() method relied on the presence of "pool" in the
text returned from toString().

This commit fixes this bug by ensuring that the text returned from
toString() contains "pool" before parsing the text. If "pool" is not
present in the text, getExecutorStatsInfo() now returns "unknown"
instead of throwing a StringIndexOutOfBoundsException.

Closes gh-27209

Closes gh-26631

Closes gh-27204

Closes gh-27206

Closes gh-27196

A side effect of 71995a90 caused the
dynamic table of contents in the left-hand side navigation to no longer
be displayed, likely due to the missing "details" DIV.

This commit addresses this issue by applying the custom header only to
index.adoc.

Consequently, the TOC is now displayed again on all pages except the
index page, but the customized header is no longer applied to those
pages.

We may revisit this issue if we decide that we want the custom header
on all pages (and not just the index page).

Closes gh-27177

See gh-27097

Closes gh-27156

Closes gh-27097

Closes gh-27110

Closes gh-27158

This commit adds a note to the documentation saying that ResourceBundle
does not support multiple bundles with the same base name.

Closes gh-27038

See gh-27075

Closes gh-27153

Closes gh-27122

This commit uses the target type where getRemoteAddress and
getLocalAddress are defined.

See gh-27120

Closes gh-26719

Closes gh-27161

Closes gh-26826

Closes gh-26999
See gh-26826

Closes gh-27081

Closes gh-27027

Closes gh-27120

Closes gh-27160

See gh-27158

Closes gh-27157

* pr/27155:
  Add Javadoc since to BeanDefinitionBuilder.setSynthetic()

Closes gh-27155

See gh-27155

* pr/27151:
  Fix link to Javadoc API
  Fix link to Javadoc API

Closes gh-27151

See gh-27151