Allow summarization threshold setting to be a ratio or token count (#319220)

chat.advanced.summarizeAgentConversationHistoryThreshold now accepts either a ratio of the model's context window (0 < value <= 1, e.g. 0.8 for 80%) or an absolute token count (value >= 100). A new resolveSummarizeThresholdTokens() helper resolves the value; ratios track model/context-size changes automatically. Values in the ambiguous (1, 100) gap are rejected so a typo like 80 (meant as 80%) fails loudly instead of compacting after 80 tokens.