[Divyanshu-2907]

Client#initialize deletes :timeout and :persistent straight from the hash the caller passes in. That causes two problems.

If you build two clients from the same config hash, the second one silently loses those settings and falls back to a 120s timeout and a persistent socket:

config = { engine: 'google', api_key: 'xxx', timeout: 5, persistent: false }
a = SerpApi::Client.new(config)
b = SerpApi::Client.new(config)
b.timeout    # => 120, expected 5
b.persistent # => true, expected false
config       # => { engine: 'google', api_key: 'xxx' }

And a frozen config hash (common for constants) raises:

SerpApi::Client.new({ engine: 'google', timeout: 5 }.freeze)
# => FrozenError: can't modify frozen Hash

The fix is to work on params.dup and take :timeout and :persistent out of the copy. I used dup rather than clone on purpose, because clone keeps the frozen flag and would still raise on a frozen hash. @params is still frozen at the end like before, so nothing changes for callers who pass a fresh hash.

I added spec/serpapi/client/constructor_spec.rb. Those specs only build clients and never hit the API, so they run without a real key.

PR #22 touched this area too, but it was closed along with a lot of other changes. This one is just the constructor fix, so it's easy to review on its own.