Retain live statistics samples through chart trimming

[stoicAI1776]

Description

Fixes #9477.

Live chart trimming keeps streamed/displayed charts bounded to the recent window, but statistics were generated from those same trimmed series. After a few days, daily Return and Benchmark samples could be trimmed down to too few points, causing PSR and related live-mode risk metrics to collapse to defaults.

This change keeps a small live-only retention buffer for the daily statistics inputs and merges those retained samples into the chart clones used for statistics generation. The visible/live chart trimming path remains unchanged.

Changes

• Made BaseResultsHandler.GenerateStatisticsResults(Dictionary<string, Chart>, ...) virtual so live results can prepare statistics inputs before delegating.
• Retain daily live samples for:
  • Strategy Equity / Equity
  • Strategy Equity / Return
  • Benchmark / Benchmark
  • Portfolio Turnover / Portfolio Turnover
• Merge retained samples into the cloned charts used for statistics, deduplicating by timestamp.
• Added coverage that simulates chart trimming and verifies statistics samples are restored before generation.

Testing

• git diff --check
• Attempted: dotnet test Tests\QuantConnect.Tests.csproj --filter RetainsDailyStatisticsSamplesAfterChartTrimming --no-restore --logger "console;verbosity=minimal"
• Attempted: dotnet build Tests\QuantConnect.Tests.csproj --no-restore -v:minimal /m:1 /nr:false
• Result: both local .NET commands hung during startup/build without compiler or fixture output and were terminated after timeout.