Fix HTTP response charset parameter parsing

[hutiefang76]

Describe what this PR does / why we need it

Decode HTTP response bodies using the declared charset for Content-Type values such as text/plain;charset=ISO-8859-1, charset="ISO-8859-1" and a charset followed by other parameters. Splitting the header on spaces either misses the charset or passes quotes/semicolons to Charset.forName.

Does this pull request fix one issue?

Fixes #3652.

Describe how you did it

Split media-type parameters on semicolons outside quoted values, respect quoted-pair escapes while splitting, and match the charset parameter case-insensitively. Reset to the configured default when no usable charset is declared. Unsupported charsets still raise the existing explicit exception.

Describe how to verify it

• JDK 17: mvn -pl sentinel-transport/sentinel-transport-simple-http -am -Dtest=SimpleHttpResponseTest,HttpEventTaskTest,CommandCenterTest -Dsurefire.failIfNoSpecifiedTests=false verify — 19 tests passed.
• JDK 8 toolchain: SimpleHttpResponseTest — 10 tests passed.
• Tests cover omitted/quoted/no-space/mixed-case/trailing parameters, semicolons inside quoted values, mutated headers, non-ASCII bodies and unsupported charsets.

Special notes for reviews

Closed PR #1590 only added null checking; this change addresses parameter parsing. No new dependency or configuration option.

AI assistance: DeepSeek and Codex assisted implementation, tests and this description.