feat: add coder_workspace_read_file MCP tool (#19562)

Follows similarly to the bash tool (and some code to connect to an agent
was extracted from it).

There are two main parts: a new agent endpoint, and then a new MCP tool
that consumes that endpoint.

coderd/httpapi/queryparams.go

@@ -120,6 +120,27 @@ func (p *QueryParamParser) PositiveInt32(vals url.Values, def int32, queryParam
 	return v
 }
 
+// PositiveInt64 function checks if the given value is 64-bit and positive.
+func (p *QueryParamParser) PositiveInt64(vals url.Values, def int64, queryParam string) int64 {
+	v, err := parseQueryParam(p, vals, func(v string) (int64, error) {
+		intValue, err := strconv.ParseInt(v, 10, 64)
+		if err != nil {
+			return 0, err
+		}
+		if intValue < 0 {
+			return 0, xerrors.Errorf("value is negative")
+		}
+		return intValue, nil
+	}, def, queryParam)
+	if err != nil {
+		p.Errors = append(p.Errors, codersdk.ValidationError{
+			Field:  queryParam,
+			Detail: fmt.Sprintf("Query param %q must be a valid 64-bit positive integer: %s", queryParam, err.Error()),
+		})
+	}
+	return v
+}
+
 // NullableBoolean will return a null sql value if no input is provided.
 // SQLc still uses sql.NullBool rather than the generic type. So converting from
 // the generic type is required.