WebFlux RequestContext should support responseEncodedHtmlEscape like Spring MVC

Background

Spring MVC's RequestContext has had two HTML escape flags since Spring 4.1.2 (see SPR-12350 / #16955):

• defaultHtmlEscape — the master switch; controls whether escaping happens at all
• responseEncodedHtmlEscape — controls which HtmlUtils.htmlEscape() overload is used when escaping is active

PR #36400 (7.0.6) wired defaultHtmlEscape into WebFlux: WebHttpHandlerBuilder → HttpWebHandlerAdapter → ServerWebExchange.HTML_ESCAPE_ATTRIBUTE → WebFlux RequestContext. That closes half the story. responseEncodedHtmlEscape was not included and remains completely absent from WebFlux.

What responseEncodedHtmlEscape does

When defaultHtmlEscape=true, the two flags combine as follows (from HtmlEscapingAwareTag):

if (isHtmlEscape()) {
    if (isResponseEncodedHtmlEscape()) {
        out = HtmlUtils.htmlEscape(content, response.getCharacterEncoding()); // encoding-aware
    } else {
        out = HtmlUtils.htmlEscape(content); // ISO-8859-1, escapes everything
    }
}

The difference matters in practice. Given the string "café" with a UTF-8 response:

With a UTF-8 response, the encoding-aware path only escapes the 5 XML-significant characters and lets native UTF-8 characters pass through unchanged. The one-arg overload converts them to HTML entities unnecessarily. MVC's default for responseEncodedHtmlEscape falls back to true, so MVC apps automatically get the correct behavior. WebFlux apps with defaultHtmlEscape=true currently always take the ISO-8859-1 path because the second flag does not exist.

Proposed solution

Mirror the exact pattern from PR #36400 across the same four files:

1. ServerWebExchange — add constant RESPONSE_ENCODED_HTML_ESCAPE_ATTRIBUTE

2. WebHttpHandlerBuilder — add field and builder method responseEncodedHtmlEscape

3. HttpWebHandlerAdapter — add field, setter, and stamp onto exchange in createExchange()

4. WebFlux RequestContext — add field, read from exchange in constructor, expose isResponseEncodedHtmlEscape() and getResponseEncodedHtmlEscape(), and use an encoding-aware escape helper

The one difference from MVC is how the response charset is resolved. MVC calls response.getCharacterEncoding(). In WebFlux the equivalent is exchange.getResponse().getHeaders().getContentType().getCharset() with a UTF-8 fallback, which is consistent with how charset is resolved elsewhere in the framework. The default for isResponseEncodedHtmlEscape() falls back to true when unset, matching MVC exactly. No breaking change.

I implemented the defaultHtmlEscape wiring in PR #36400 and I'm prepared to open a PR and implement this follow-up across the same files

Start by comparing the four WebFlux files named in the issue with the wiring pattern from PR #36400: ServerWebExchange, WebHttpHandlerBuilder, HttpWebHandlerAdapter, and WebFlux RequestContext. Done means the new flag flows through the exchange, exposes both accessors, uses the response charset with UTF-8 fallback, and preserves MVC's true default behavior.