feat(chat): Remove thinking XML tags and add thinking content part

[abeatrix]

This commit introduces the following changes:

• Removes the XML tags from the thinking text in CompletionsResponseBuilder.ts.
• Adds a ThinkingContentPart type to lib/shared/src/sourcegraph-api/completions/types.ts to represent the thinking content.
• Updates lib/shared/src/sourcegraph-api/completions/parse.ts to handle the delta_thinking event and add the thinking content to the content array.
• Updates vscode/src/prompt-builder/sanitize.ts to remove content between <think> tags from the first human message.

These changes allow the client to handle the thinking content separately from the completion text, and remove the need for XML tags.

Test plan

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