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

### Under which category would you file this issue?

Providers

### Apache Airflow version

main (development)

### What happened and how to reproduce it?

`HttpHook._url_from_endpoint()` (in `providers/http/src/airflow/providers/http/hooks/http.py`) only guards against a *missing* slash between `base_url` and `endpoint` — it doesn't guard against a *double* one. If a connection's host is configured with a trailing `/` (common) and an endpoint is passed with a leading `/` (also common REST convention), the resulting URL has `//` in it.

Repro:

```python
from airflow.providers.http.hooks.http import HttpHook

hook = HttpHook()
hook.base_url = "https://api.example.com/v1/"
hook._base_url_initialized = True
print(hook.url_from_endpoint("/users"))
# https://api.example.com/v1//users <- double slash
```

Many API frameworks (Flask, FastAPI, Django) do not normalize `//` in a path and will 404 on it, so this produces a silent, confusing failure for anyone hitting this combination. Affects both `HttpHook` and `HttpAsyncHook`, since both call the shared `_url_from_endpoint()` helper.

### What you think should happen instead?

Exactly one `/` should separate the base URL and endpoint regardless of which side(s) already have a slash. `_url_from_endpoint()` should strip the trailing slash from `base_url` and the leading slash from `endpoint` before joining, rather than only checking for the missing-slash case.

Happy to open a PR with the fix and a test covering this slash combination — will link it here.

### Operating System

_No response_

### Deployment

None

### Apache Airflow Provider(s)

http

### Versions of Apache Airflow Providers

_No response_

### Official Helm Chart version

Not Applicable

### Kubernetes Version

_No response_

### Helm Chart configuration

_No response_

### Docker Image customizations

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start in providers/http/src/airflow/providers/http/hooks/http.py at HttpHook._url_from_endpoint(), then check how HttpAsyncHook uses the shared helper. Reproduce the trailing-slash and leading-slash combination from the issue, and verify the resulting URL has exactly one separator while existing missing-slash behavior remains correct.

Written by the indexing model from the issue text.