fix: listTasks query parsing

[nahapetyan-serob]

GET /tasks?historyLength=N and ?statusTimestampAfter=... were silently
dropped by the REST ListTasks handler. The parse closure dispatched on
target type with a type switch, but *int / *time.Time fields on
ListTasksRequest produced **int / **time.Time arguments that no case
matched — so the values never reached the request.

Replaced the type-switch parse closure with a typed parseQueryParams
helper that returns (*a2a.ListTasksRequest, error).
Extracted parseHistoryLength and parseStatusTimestampAfter helpers,
reused by handleGetTask and handleListTasks.

Added new test to verify correctly parsed ListTasksRequest reaching to handler
Extended TestREST_ListTasksParseErrors with historyLength and
statusTimestampAfter invalid-value cases.

[gemini-code-assist]

Code Review

This pull request refactors query parameter parsing in a2asrv/rest.go by replacing inline parsing logic with dedicated helper functions (parseListTasksQueryParams, parseHistoryLength, and parseStatusTimestampAfter) and adds comprehensive unit tests in a2asrv/rest_test.go to verify query parsing. The reviewer suggested simplifying the parseHistoryLength and parseStatusTimestampAfter helper functions by returning early when the query parameter is empty, which improves readability and avoids declaring unnecessary pointer variables.

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[gemini-code-assist]

This helper function can be simplified by returning early when the query parameter is empty. This avoids declaring an unnecessary pointer variable and makes the logic more direct and readable.

func parseHistoryLength(query url.Values) (*int, error) {
	raw := query.Get("historyLength")
	if raw == "" {
		return nil, nil
	}
	val, err := strconv.Atoi(raw)
	if err != nil {
		return nil, err
	}
	return &val, nil
}

[gemini-code-assist]

This helper function can be simplified by returning early when the query parameter is empty. This avoids declaring an unnecessary pointer variable and makes the logic more direct and readable.

func parseStatusTimestampAfter(query url.Values) (*time.Time, error) {
	raw := query.Get("statusTimestampAfter")
	if raw == "" {
		return nil, nil
	}
	val, err := time.Parse(time.RFC3339Nano, raw)
	if err != nil {
		return nil, err
	}
	return &val, nil
}