This commit better aligns how URI variable placeholders are detected
in UriComponentsBuilder#encode (i.e. the pre-encoding of the literal
parts of a URI template) and how they are expanded later on.
The latter relies on a pattern that stops at the first closing '}'
which excludes the possibility for well-formed, nested placeholders
other than variables with regex syntax, e.g. "{year:\d{1,4}}".
UriComponentsBuilder#encode now also stops at the first closing '}' and
further ensures the placeholder is not empty and that it has '{' before
deciding to treat it as a URI variable.
Closes gh-26466
8791928f61