feat(router): add preserve_encoded_slash to keep %2F in path parameters

[AlinsRan]

Description

Nginx decodes an URL-encoded slash (%2F) into a real / in $uri before
route matching happens. As a result a request like
/v1/te%2Fst/products/electronics/list is seen by the router as
/v1/te/st/products/electronics/list, which has a different number of path
segments and therefore fails to match a route defined with path parameters such
as /v1/:id/products/:type/list (returns 404 Route Not Found). This is the
problem reported in #11810, and it cannot be fixed in lua-resty-radixtree
(the parameter pattern [^/]+ must stop at a real slash to keep segmentation
correct) because the information is already lost by the time the router runs.

This PR adds an opt-in apisix.preserve_encoded_slash option (default false).
When enabled, the route matching uri is rebuilt from the raw request_uri with
every percent-encoding decoded except %2F/%2f, which is kept encoded so
it is treated as part of a path parameter instead of a separator. The encoded
slash is then forwarded to the upstream as is.

To stay safe while bypassing Nginx's $uri normalization, the rebuild:

• resolves . and .. segments (so an encoded dot like %2e%2e cannot be used
  to bypass route rules via path traversal);
• rejects a decoded null byte, matching Nginx's own behaviour;
• only runs when an encoded slash is actually present, so requests without %2F
  keep using the already normalized $uri with no overhead.

This follows the same access-phase, opt-in pattern as the existing
normalize_uri_like_servlet option.

Which issue(s) this PR fixes:

Fixes #11810

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (the option defaults to false)