fix: snapshot watch results before rendering

[skt-shinyruo]

Summary

• render watch expression results at advice time and store the rendered snapshot on ObjectVO
• make terminal, HTTP API, and MCP ObjectVO rendering prefer the stored watch snapshot
• add regression coverage for mutable AtEnter arguments and MCP renderedValue handling

Test Plan

• ./mvnw -ntp -pl core -am -Dtest=WatchAdviceListenerTest,McpObjectVOFilterTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -ntp -pl core -am test
• git diff --check

Notes

• Full local reactor check passed with 263 tests, 0 failures, 0 errors, 7 skipped.