fix(sequentialthinking): accurate readOnlyHint and idempotentHint annotations (#4721)

[teddiesloco]

Summary

• Fixes sequential-thinking: readOnlyHint and idempotentHint annotations are inaccurate (server is stateful, non-idempotent) #4721
• Corrects readOnlyHint from true to false because SequentialThinkingServer actively mutates session state (thoughtHistory and branches array) on each invocation.
• Corrects idempotentHint from true to false because repeated invocations with the same inputs produce different thoughtHistoryLength values and branch mutations.
• Both hints now accurately reflect the stateful runtime nature of the sequentialthinking tool.