AI Gateway custom providers appear to prepend `/v1` to provider-specific endpoint paths

When using an AI Gateway custom provider via the provider-specific endpoint, Gateway appears to prepend `/v1` to the upstream path even when the custom provider `base_url` is configured without `/v1`.

Per the docs, this route:

`https://gateway.ai.cloudflare.com/v1/{account_id}/{gateway_id}/custom-{slug}/{provider-path}`

should map to:

`{base_url}/{provider-path}`

Docs reference:
[Custom Providers ->Provider with a non-standard API path][(https://developers.cloudflare.com/ai-gateway/configuration/custom-providers/#via-provider-specific-endpoint](https://developers.cloudflare.com/ai-gateway/configuration/custom-providers/#example-2-provider-with-a-non-standard-api-path))

In my case, the custom provider is configured as:

- `slug`: `exa-provider`
- `base_url`: `https://api.exa.ai`

So this Gateway request:

`POST .../custom-exa-provider/search`

should go upstream to:

`POST https://api.exa.ai/search`

But the actual upstream behavior looks like:

`POST https://api.exa.ai/v1/search`

## Steps to reproduce

1. Create a custom provider with:
- `slug`: `exa-provider`
- `base_url`: `https://api.exa.ai`
2. Create a provider config / BYOK alias for that provider.
3. Send this request through AI Gateway:

```bash
curl -sS -i -X POST \
"https://gateway.ai.cloudflare.com/v1/${ACCOUNT_ID}/${GATEWAY_ID}/custom-exa-provider/search" \
-H "cf-aig-authorization: Bearer ${AI_GATEWAY_TOKEN}" \
-H "cf-aig-byok-alias: ${BYOK_ALIAS}" \
-H "Content-Type: application/json" \
-d '{"query":"Satya Nadella","category":"people","numResults":2}'

Start by tracing the provider-specific custom route described in the issue and how it combines the configured base_url with the provider path. Reproduce the curl request with a base_url of https://api.exa.ai, then verify that the upstream request uses /search rather than /v1/search.

Written by the indexing model from the issue text.