Fix context key collision: agentSessionType and chatSessionType

[Copilot]

Both ChatContextKeys.chatSessionType and ChatContextKeys.agentSessionType were using the same underlying context key string 'chatSessionType', causing ambiguity in context key resolution.

Changes

Changed agentSessionType to use distinct key string:

// Before
export const agentSessionType = new RawContextKey<string>('chatSessionType', '', ...);

// After  
export const agentSessionType = new RawContextKey<string>('agentSessionType', '', ...);

All usages already reference the context key through the API (.key, .isEqualTo(), .bindTo()), so no consumer changes required.

Original prompt

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This section details on the original issue you should resolve

<issue_title>There's a naming collision: both ChatContextKeys.chatSessionType (line 65) and ChatContextKeys.agentSessionType (line 103) use the same context key string 'chatSessionType'. This creates ambiguity and can lead to unexpected behavior. The agentSessionType export should use a different key string, such as 'agentSessionType'.</issue_title>
<issue_description>There's a naming collision: both ChatContextKeys.chatSessionType (line 65) and ChatContextKeys.agentSessionType (line 103) use the same context key string 'chatSessionType'. This creates ambiguity and can lead to unexpected behavior. The agentSessionType export should use a different key string, such as 'agentSessionType'.

	export const chatSessionType = new RawContextKey<string>('sessionType', '', { type: 'string', description: localize('chatSessionType', "The type of the current chat session.") });

Originally posted by @copilot in #288181 (comment)
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes There's a naming collision: both ChatContextKeys.chatSessionType (line 65) and ChatContextKeys.agentSessionType (line 103) use the same context key string 'chatSessionType'. This creates ambiguity and can lead to unexpected behavior. The agentSessionType export should use a different key string, such as 'agentSessionType'. #288692

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