[apiview-stub-generator] Follow-ups from #16395: core install timeout + simplify feed index handling

Follow-ups from unresolved review comments on #16395 (apiview-stub-generator package-install handling). #16395 dropped the timeout/env-var change; these items remain open.

## 1. Investigate `azure-ai-agentserver-core` install timeout (azure-sdk-for-python)

The 120s→300s timeout in #16395 was a symptom of PyPI egress/auth being blocked; the real fix is the `auth-dev-feed` / `PIP_INDEX_URL` plumbing, so the timeout change was reverted. **But** in azure-sdk-for-python — where feed auth is already in place — `azure-ai-agentserver-core` still times out at exactly 120s on `pip install `, while its 4 siblings install in 15–26s:

| package | duration |
|---|---|
| activity | 24s |
| invocations | 22s |
| optimization | 16s |
| responses | 26s |
| **core** | **FAIL @ 133s** |

This is a genuine install-duration problem for core's dependency tree (opentelemetry distro), not a network symptom. Investigate why core resolves/installs so slowly and how to fix it (e.g. constrain/pin the opentelemetry dep set, prebuilt wheel reuse).

Reference: [build 6577980](https://dev.azure.com/azure-sdk/internal/_build/results?buildId=6577980&view=logs&j=e5529761-e01f-5e88-6f54-d815165135e2&t=046b8c0d-ddf8-57b5-04c7-7c9a5017983e&s=f283cc76-abfa-545b-e138-bb740b7a9267)
From: https://github.com/Azure/azure-sdk-tools/pull/16395#discussion_r3617342794

## 2. Drop redundant `--index-url` logic in `_download_package_from_index`

`tests/azure_tests/azure_test.py` builds an explicit `--index-url` and passes it to `pip download`. Since pip already honors `PIP_INDEX_URL` from the environment (set by PipAuthenticate in CI), the explicit index handling may be unnecessary. Relying on `PIP_INDEX_URL` decouples us from deciding which index to download from.

Investigate: can we drop the explicit `--index-url`/default-fallback and let pip use `PIP_INDEX_URL`?

From: https://github.com/Azure/azure-sdk-tools/pull/16395#discussion_r3618192176

## 3. Confirm DevOps feed endpoint works for direct file download

Related to #2: does the DevOps feed expose the same endpoint pip expects? If so, building the URL from `PIP_INDEX_URL` and keeping the logic the same would be simpler — but it's unclear whether `pip download` will actually pull the artifact through the feed. Verify feed-backed download behavior before simplifying.

From: https://github.com/Azure/azure-sdk-tools/pull/16395#discussion_r3618205263

Start by reviewing build 6577980 and the azure-ai-agentserver-core installation path, comparing it with the four sibling packages and their dependency trees. Then inspect tests/azure_tests/azure_test.py and verify whether pip download works through the DevOps feed using PIP_INDEX_URL. Done means the core timeout has a documented fix or cause, and the index-handling decision is validated by tests or a confirmed feed download.

Written by the indexing model from the issue text.