fix: pin Pygments <2.20 to fix docs build (#185)

## Summary

- Pins Pygments to `>=2.19.2,<2.20` in the docs dependency group to
avoid a breaking change in Pygments 2.20.0
- Pygments 2.20.0 (bumped in #182) introduced a regression in
`HtmlFormatter.__init__` where `_decodeifneeded()` returns `None`,
causing `pymdown-extensions` to crash with `AttributeError: 'NoneType'
object has no attribute 'replace'` when rendering function signatures
via `mkdocstrings`
- This broke the "Build documentation" step in the Release workflow
([failed
run](https://github.com/scaleapi/scale-agentex/actions/runs/23954964095/job/69871063041))

## Test plan

- [x] `mkdocs build` succeeds locally with Pygments 2.19.2
- [ ] CI Release workflow passes on merge

<!-- greptile_comment -->

<h3>Greptile Summary</h3>

This PR is a targeted dependency fix that pins `Pygments` to
`>=2.19.2,<2.20` in the `docs` dependency group to unblock the docs
build broken by a regression introduced in Pygments 2.20.0. The fix is
straightforward and correct for its stated purpose.

- Adds `Pygments>=2.19.2,<2.20` to the `docs` dependency group in
`pyproject.toml` and updates `uv.lock` accordingly.
- The upper-bound pin (`<2.20`) is conservative — using `!=2.20.0` would
be less restrictive and would automatically pick up any 2.20.x patch
that fixes the upstream regression.
- `uv.lock` also contains unrelated additions for
`claude_agent_sdk-0.1.49` (a new `sdist` entry and an `x86_64` wheel)
that appear to have been swept in when `uv lock` was re-run; these are
harmless but worth noting.
- A pre-existing duplicate of `agentex-sdk` exists in the `docs`
dependency group (lines 60 and 64) — this was not introduced by this PR.

<details><summary><h3>Confidence Score: 5/5</h3></summary>

Safe to merge — the change is a minimal, targeted dependency pin that
unblocks the docs build with no impact on runtime code.

The change only affects the `docs` dependency group, has no impact on
production or test code, and is well-explained. The only minor concerns
are the choice of `<2.20` vs `!=2.20.0` and the unrelated lockfile
entries, neither of which is a blocker.

No files require special attention; both changed files are
dependency-management artifacts.
</details>

<h3>Important Files Changed</h3>

| Filename | Overview |
|----------|----------|
| agentex/pyproject.toml | Adds `Pygments>=2.19.2,<2.20` to the docs
dependency group to work around the 2.20.0 regression; note that
`agentex-sdk` is listed twice in this group (pre-existing duplicate). |
| uv.lock | Lockfile updated to reflect the Pygments constraint; also
includes unrelated sdist and x86_64 wheel additions for
claude_agent_sdk-0.1.49 that were swept in by re-running uv lock. |

</details>

<details><summary><h3>Flowchart</h3></summary>

```mermaid
%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[mkdocs build] --> B{Pygments version}
    B -- "2.20.0 (broken)" --> C["HtmlFormatter.__init__\n_decodeifneeded() → None"]
    C --> D["AttributeError: 'NoneType'\n.replace() in pymdown-extensions"]
    D --> E[Build fails ❌]
    B -- ">=2.19.2,<2.20 (pinned)" --> F[HtmlFormatter works correctly]
    F --> G[mkdocstrings renders function signatures]
    G --> H[Build succeeds ✅]
```
</details>

<details><summary>Prompt To Fix All With AI</summary>

`````markdown
This is a comment left during a code review.
Path: agentex/pyproject.toml
Line: 63

Comment:
**Consider `!=2.20.0` instead of `<2.20`**

The PR description says the regression is specifically in Pygments 2.20.0. Pinning `<2.20` will also block any future 2.20.x patch releases that might fix the issue (e.g. 2.20.1). Using `!=2.20.0` would be less restrictive and would allow a future patch fix to be picked up automatically.

```suggestion
    "Pygments>=2.19.2,!=2.20.0",
```

If the intent is to stay cautious until upstream confirms a fix, the current `<2.20` pin is acceptable — but worth revisiting once the Pygments team releases a patch.

How can I resolve this? If you propose a fix, please make it concise.

---

This is a comment left during a code review.
Path: uv.lock
Line: 521-527

Comment:
**Unrelated `claude_agent_sdk` lockfile changes**

The diff includes an added `sdist` entry and a new `x86_64` wheel for `claude_agent_sdk-0.1.49` that are unrelated to the Pygments pin. These appear to have been swept in when `uv lock` was re-run. While this is harmless, mixing unrelated lockfile changes makes the PR harder to audit. Consider either reverting those hunks or noting why they are included.

How can I resolve this? If you propose a fix, please make it concise.
`````

</details>

<sub>Reviews (1): Last reviewed commit: ["fix: pin Pygments &lt;2.20 to
fix docs
buil..."](b9c20a2)
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Greptile](https://app.greptile.com/api/retrigger?id=27296664)</sub>

<!-- /greptile_comment -->

Author: smoreinis

agentex/pyproject.toml

@@ -60,6 +60,7 @@ docs = [
     "agentex-sdk",
     "griffe-pydantic>=1.1.4",
     "pymdown-extensions>=10.0,<11",
+    "Pygments>=2.19.2,<2.20",
     "agentex-sdk",
 ]