[On-Hold] Move LLM prompt templates from embedded constants to resource files

[Copilot]

Addresses review feedback from #318 to externalize large prompt template strings that were bloating the extension bundle.

Changes

Resource Files

• Created resources/prompts/ with 5 markdown template files (~30KB total)
• Templates: find-query, aggregate-query, count-query, cross-collection-query, single-collection-query

Template Loading

• Added PromptTemplateLoader utility with lazy loading and in-memory caching
• Replaced string constants with async getter functions in promptTemplates.ts
• Updated PromptTemplateService to handle async template loading

Impact

• Reduced promptTemplates.ts from 517 to 166 lines (-393 lines)
• Templates no longer compiled into bundle, loaded on-demand from disk
• Zero breaking changes: async functions replace constants at call sites

Example

Before:

export const FIND_QUERY_PROMPT_TEMPLATE = `
You are an expert MongoDB assistant...
[400+ lines of template content]
`;

After:

export async function getFindQueryPromptTemplate(): Promise<string> {
    return promptTemplateLoader.loadTemplate('find-query-prompt-template.md');
}

Usage in promptTemplateService.ts:

// Now async
private static async getBuiltInQueryGenerationTemplate(type: QueryGenerationType): Promise<string> {
    switch (type) {
        case QueryGenerationType.CrossCollection:
            return getCrossCollectionQueryPromptTemplate();
        // ...
    }
}

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