feat: expose stream logs configuration in gateway chart

[Copilot]

Add stream access log settings to values.yaml and wire them into the configmap template, mirroring the existing HTTP logs pattern.

• values.yaml: New logs.stream section with enableAccessLog, accessLog, accessLogFormat, accessLogFormatEscape
• configmap.yaml: Render stream log fields under nginx_config.stream, including fallback_cp write-mode guard that forces enable_access_log: false
• Chart.yaml: Bump version 0.2.50 → 0.2.51
• README.md: Regenerated via make helm-docs

Default stream config:

logs:
  stream:
    enableAccessLog: false
    accessLog: "logs/access_stream.log"
    accessLogFormat: "$remote_addr [$time_local] $protocol $status $bytes_sent $bytes_received $session_time $apisix_request_id"
    accessLogFormatEscape: default

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