Claude agent: add auto permission mode + non-streaming passthrough (#316128)

* Claude agent: add auto permission mode + non-streaming passthrough

- Adds 'auto' permission mode (model-classifier) to Claude sessions, gated
  by new setting github.copilot.chat.claudeAgent.allowAutoPermissions
  (default true).
- Threads allowAutoPermissions through ClaudeSessionOptionBuilder and the
  reactive options pipeline in ClaudeChatSessionContentProvider.
- ClaudeLanguageModelServer: defer response headers until upstream content-type
  is known and propagate the SDK's stream flag via requestOptions, so the
  classifier's non-streaming (application/json) calls used by 'auto' mode
  pass through correctly alongside the existing SSE path.

* add non-streaming passthrough response handling in ClaudeLanguageModelServer

* Claude auto mode: mark setting preview, link aka.ms, honor preview features policy

- Move github.copilot.chat.claudeAgent.allowAutoPermissions into the 'preview' config section and tag it with 'preview'.
- Description now links to https://aka.ms/vscode-claude-auto-mode.
- Auto mode is gated on copilotToken.isEditorPreviewFeaturesEnabled() in addition to the setting, so org-level preview-features policy disables it.

* Claude auto mode: default false, ExperimentBased + onExp tag

- Default off; flipped via experiment.
- Setting tagged with both 'preview' and 'onExp'.
- ConfigKey.ClaudeAgentAllowAutoPermissions is now ExperimentBased; option builder and content provider read it via getExperimentBasedConfig with the injected IExperimentationService.

* Claude auto mode: default to true in the agents window

* Revert "Claude auto mode: default to true in the agents window"

This reverts commit 9083b1c2fd177767b59be6886a2510a9d8309db5.

* Sessions/Claude: add 'Auto' permission mode item to picker

* Sessions/Claude: gate Auto permission mode item on allowAutoPermissions setting

* Sessions/Claude: also gate Auto on chat entitlement preview features

* Sessions/Claude: use observableConfigValue for allowAutoPermissions

* Sessions/Claude: append Auto mode when enabled instead of filtering

* Sessions/Claude: stub IConfigurationService and IChatEntitlementService in picker tests

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Co-authored-by: bhavyaus <bhavyau@microsoft.com>