fix: use percent-placeholder args in HTTPError to prevent Tornado from doubling percent signs in gateway URLs

[terminalchai]

Problem

Tornado's web.HTTPError escapes % as %% in the log_message string when no positional args are provided. This is by design — Tornado passes log_message % args through Python's %-formatting, so any literal % in the message must be doubled to survive.

The three HTTPError raises in send_request_to_gateway() used f-strings as log_message with no args. When the gateway URL contains percent-encoded characters like http%3A%2F%2F, Tornado's escaping doubled every %:

"ConnectionError was received from Gateway server url 'http%%3A%%2F%%2Fexample.com'."

Closes #1503.

Fix

Replace f-string log_message arguments with %s placeholder strings and pass the URL/message values through the args parameter. Tornado only escapes log_message when args is empty, so URL percent characters are preserved correctly:

# Before
raise web.HTTPError(503, f"...url '{gateway_client.url}'.")

# After
raise web.HTTPError(503, "...url '%s'.", gateway_client.url)

Applied to all three HTTPError raises in gateway_request().

Test

Added test_gateway_httperror_percent_not_doubled() that:

1. Creates an HTTPError the old way (f-string, no args) and asserts Tornado does double % — proves the old code was buggy.
2. Creates an HTTPError the new way (%s placeholder + args) and asserts %% is not present in log_message and the URL is intact in args.

[Carreau]

I this usually the '%s' is a code small that one should use %r instead but it's pure improvement