Make control plane base_uri configurable for Pinecone Local support

[ScotterC]

Summary

The control plane (Index, Collection) hardcoded https://api.pinecone.io, making it impossible to use Pinecone Local's database emulator for index management without monkey-patching.

This PR makes base_uri configurable with a default of https://api.pinecone.io — fully backward compatible for cloud users, and now usable with local containers.

The data plane (Vector) already supports http:// prefixes on host parameters, so no changes are needed there. Users just need to pass host: "http://dense-index:5081" instead of host: "dense-index:5081".

Usage

# Cloud (default, nothing changes)
Pinecone.configure do |config|
  config.api_key = "pc-real-key"
end

# Local development
Pinecone.configure do |config|
  config.api_key = "anything"
  config.base_uri = "http://localhost:5080"  # control plane
end
client.index(host: "http://localhost:5081")  # data plane (already worked)

Changes

• Configuration#base_uri defaults to https://api.pinecone.io instead of raising
• Index and Collection read base_uri from config instead of hardcoding
• Removed define_singleton_method monkey-patch from test helpers

Closes #47, closes #51

Test plan

• All specs pass against local Pinecone containers
• StandardRB lint clean
• Verify CI passes with container-based tests

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