[collector]bugfix: honor XML response charset

[hutiefang76]

What's changed

Close #2852.

• Decode XML HTTP responses with the charset from Content-Type when present.
• Fall back to the XML declaration charset before using UTF-8.
• Keep non-XML response parsing behavior unchanged.
• Add regression coverage for ISO-8859-1 XML without an HTTP charset and unsupported XML charset fallback.

Verification

• JAVA_HOME=$(/usr/libexec/java_home -v 25 2>/dev/null || /usr/libexec/java_home) ./mvnw -pl hertzbeat-collector/hertzbeat-collector-basic -am -Dtest=HttpCollectImplTest -Dsurefire.failIfNoSpecifiedTests=false test
• JAVA_HOME=$(/usr/libexec/java_home -v 25 2>/dev/null || /usr/libexec/java_home) ./mvnw -pl hertzbeat-collector/hertzbeat-collector-basic -am test
• git diff --check