fix: correct off-by-one error in hook template message limit enforcement (#7005)

[vigneshakaviki]

Problem

The hook template message limit is incorrectly enforced by checking newline count instead of actual message count. This allows templates with 11 messages to bypass the 10-message limit.

Root Cause

Current implementation:

if n := strings.Count(out, "\n"); n > maxMessages {
    return nil, fmt.Errorf(...)
}
return strings.SplitN(out, "\n", maxMessages), nil

The check counts newlines, but:

• 10 newlines = 11 messages (lines)
• Condition 10 > 10 is false, so 11-message output passes
• SplitN(..., 10) then merges the 11th message into element 10 as embedded newline

Impact

The "maximum 10 messages" guard can be bypassed by one extra rendered line, allowing hook outputs larger than intended.

Solution

Replace newline counting with actual message splitting:

• Split output first
• Check length of resulting slice
• Return the split messages directly

This correctly enforces the limit on the actual number of messages.

Testing

Added comprehensive test cases:

• Exactly 10 messages ✓
• 10 lines with trailing newline ✓
• 11 lines (off-by-one case) - now correctly rejected ✓
• 12+ lines - correctly rejected ✓
• Single message - correctly accepted ✓

Closes #7005