Fix HttpHook double slash in URL when connection host has trailing slash and endpoint has leading slash

[aryansk]

closes #71658

What is changing and why

HttpHook._url_from_endpoint() (used by both HttpHook and HttpAsyncHook) only guarded against a missing slash between base_url and endpoint — it did not guard against a double one. When a connection host is configured with a trailing / (common) and an endpoint is passed with a leading / (also common REST convention), the resulting URL contained //:

hook.base_url = "https://api.example.com/v1/"
hook.url_from_endpoint("/users")
# before: https://api.example.com/v1//users
# after:  https://api.example.com/v1/users

Many API frameworks (Flask, FastAPI, Django) do not normalize // in a path and return 404, so this produced a silent, confusing failure.

The fix joins base_url and endpoint with exactly one / in all cases, preserving the existing behavior for every other input combination (including scheme-only base URLs such as http://, and endpoint-only requests).

Tests

• Added parametrized regression cases to test_url_from_endpoint covering trailing-slash, leading-slash, and both.
• providers/http/tests/unit/http/hooks/test_http.py: 60 passed, 1 pre-existing environment error (verified identical on main).
• ruff check and ruff format --check clean on both changed files.

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