Validate markdown table separators before buffering in stream handler

[Copilot]

The stream handler buffered any line starting with | as table content, delaying output of non-table content and degrading UX during streaming.

Changes:

• Table separator validation: Added regex pattern to verify markdown table separator lines (e.g., | --- | :---: | ---: |) before treating content as a table
• Two-line lookahead: Buffer first |-prefixed line, verify second line is a valid separator, then continue buffering or flush as non-table content
• State tracking: Introduced tableHeaderSeen flag to distinguish validated tables from false positives

// Before: any line starting with | was buffered
if (trimmedLine.indexOf("|") === 0) {
    tableBuffer += line + "\n"
    continue
}

// After: validate separator exists before buffering
var TABLE_SEPARATOR_REGEX = /^\s*\|(\s*:?-+:?\s*\|)+\s*$/
if (trimmedLine.indexOf("|") === 0) {
    var isSeparator = TABLE_SEPARATOR_REGEX.test(trimmedLine)
    if (!inTable) {
        // Buffer first line, wait for separator confirmation
        tableBuffer = line + "\n"
        tableHeaderSeen = false
        continue
    }
    if (!tableHeaderSeen && !isSeparator) {
        // Second line not a separator - flush as regular content
        flushContent(tableBuffer)
        flushContent(line + "\n")
        // ...reset state
    }
}

This prevents buffering of shell output, log lines, or other content that happens to start with | while preserving correct table rendering.

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