fix config: honor explicit args instead of returning cached default

[Tyagiquamar]

What changed

• upstox_client/configuration.py: TypeWithDefault.__call__ now constructs a fresh Configuration when explicit args/kwargs are passed, instead of always returning a copy of the first-cached default. Bare Configuration() calls keep the existing cached-default behavior, and ApiClient (which calls Configuration() with no args) is unaffected.
• Added test/test_configuration.py with 4 network-free unit tests.

Why

Fixes #155. Once any Configuration() was created (e.g. live), a later Configuration(sandbox=True) silently returned a copy of the live config, so sandbox endpoints were never used unless callers hand-patched host/order_host.

Testing

Docker python:3.11-bookworm:

• python -m unittest test.test_configuration — 4 passed with the fix; 3 fail on base (the sandbox-arg cases), 1 passes both (bare-call caching, by design).
• python -m compileall -q upstox_client/configuration.py test/test_configuration.py — OK.
• Note: Crash on modern Python environments due to urllib3 >= 2.0.0 deprecation #156 (urllib3.get_host) was checked while triaging — no get_host usage remains in the current tree, so only Configuration Class behaves as a Singleton, ignoring subsequent __init__ arguments #155 is addressed here.