Add dictionary parameter support for QueryParam, FormParam, and HeaderParam attributes

[Copilot]

This PR implements support for passing IDictionary parameters to QueryParamAttribute, FormParamAttribute, and HeaderParamAttribute. When a dictionary is provided, its key-value pairs are automatically expanded into multiple individual parameters.

Changes Made

• Modified RequestHandler.GetParams() to check if parameter values implement IDictionary
• When a dictionary is detected, iterates through all key-value pairs and adds them as separate parameters
• Maintains full backward compatibility - existing single-value parameters continue to work unchanged
• Added comprehensive test coverage including edge cases

Usage Examples

Query Parameters:

var filters = new Dictionary<string, string> 
{
    ["category"] = "electronics",
    ["price_min"] = "100",
    ["price_max"] = "500"
};

[Get("/search")]
Task<SearchResult> Search([QueryParam] Dictionary<string, string> filters);
// Creates: /search?category=electronics&price_min=100&price_max=500

Form Parameters:

var formData = new Dictionary<string, string>
{
    ["firstName"] = "John",
    ["lastName"] = "Doe",
    ["email"] = "john@example.com"
};

[Post("/submit")]
Task SubmitForm([FormParam] Dictionary<string, string> data);
// Sends as form-encoded data with all fields

Header Parameters:

var headers = new Dictionary<string, string>
{
    ["X-API-Version"] = "2.0",
    ["X-Client-ID"] = "app123"
};

[Get("/data")]
Task<string> GetData([HeaderParam] Dictionary<string, string> headers);
// Adds all headers to the request

Mixed Parameters:

[Get("/advanced")]
Task<Result> Search(
    [QueryParam] Dictionary<string, string> filters,
    [QueryParam("page")] int page,
    [QueryParam("limit")] int limit);
// Combines dictionary expansion with regular parameters

Features

• Automatic Expansion: Dictionary entries become individual parameters
• Null Safety: Handles null keys/values gracefully by converting to empty strings
• Proper Encoding: Query parameters are URL-encoded correctly
• Backward Compatibility: Existing code continues to work without changes
• Mixed Usage: Can combine dictionaries with regular parameters in the same method

The implementation handles edge cases like empty dictionaries and null values, ensuring robust behavior in all scenarios.

Fixes #87.

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