fix(agentex-ui): include account_id in investigate traces link (#310)

# Pull Request Summary

The AgentEx "Investigate traces" link opened SGP monitor with only trace
ID query params. When a user had multiple tenants, SGP could load traces
under the wrong active account. This change reads `account_id` from the
current AgentEx URL and appends it to the monitor link so SGP switches
to the correct tenant.

## Test Plan

- Verified the link builder adds `account_id` when the AgentEx page URL
includes `account_id`.
- Agentex UI lint passed via pre-commit on the changed file.

## Linear Issue

No Linear ticket.

Made with [Cursor](https://cursor.com)

<!-- greptile_comment -->

<h3>Greptile Summary</h3>

This PR fixes a multi-tenant issue in the "Investigate traces" button by
reading `account_id` from the current AgentEx URL and appending it to
the SGP monitor link, ensuring SGP loads the correct tenant context.

- Introduces `useSafeSearchParams` to extract `account_id` from the
current page's search params and conditionally appends it to the
outbound SGP URL.
- Refactors the URL construction to use `URLSearchParams` for cleaner
parameter handling.
- The diff also silently changes the SGP monitor path from
`/beta/monitor` to `/monitor`; this is not mentioned in the PR
description and warrants confirmation that SGP no longer requires the
`/beta/` prefix.

<details><summary><h3>Confidence Score: 4/5</h3></summary>

The account_id forwarding logic is correct and well-guarded, but the
undocumented removal of the `/beta/` path segment in the monitor URL
could silently break the link for all users if SGP has not retired that
route.

The path change from `/beta/monitor` to `/monitor` is unmentioned in the
PR description and untested in the stated test plan — if SGP still
routes the monitor behind `/beta/`, every "Investigate traces" click
will land on the wrong or missing page.

agentex-ui/components/task-header/investigate-traces-button.tsx —
confirm the `/beta/monitor` → `/monitor` path change is intentional and
that the new route is live in SGP.
</details>

<h3>Important Files Changed</h3>

| Filename | Overview |
|----------|----------|
| agentex-ui/components/task-header/investigate-traces-button.tsx |
Reads `account_id` from search params and appends it to the SGP monitor
URL; also silently changes the monitor path from `/beta/monitor` to
`/monitor` |

</details>

<details><summary><h3>Sequence Diagram</h3></summary>

<a href="#gh-light-mode-only">

```mermaid
%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant User
    participant AgentEx UI
    participant useSafeSearchParams
    participant SGP Monitor

    User->>AgentEx UI: Visits page with ?account_id=XYZ&task_id=ABC
    AgentEx UI->>useSafeSearchParams: read search params
    useSafeSearchParams-->>AgentEx UI: sgpAccountID = "XYZ"
    AgentEx UI->>AgentEx UI: Build URL: /monitor?trace_id=...&tt-trace-id=...&account_id=XYZ
    User->>SGP Monitor: Clicks "Investigate traces" link
    SGP Monitor-->>User: Opens monitor under account XYZ
```

</a>
<a href="#gh-dark-mode-only">

```mermaid
%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant User
    participant AgentEx UI
    participant useSafeSearchParams
    participant SGP Monitor

    User->>AgentEx UI: Visits page with ?account_id=XYZ&task_id=ABC
    AgentEx UI->>useSafeSearchParams: read search params
    useSafeSearchParams-->>AgentEx UI: sgpAccountID = "XYZ"
    AgentEx UI->>AgentEx UI: Build URL: /monitor?trace_id=...&tt-trace-id=...&account_id=XYZ
    User->>SGP Monitor: Clicks "Investigate traces" link
    SGP Monitor-->>User: Opens monitor under account XYZ
```

</a>
</details>

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

`````markdown
Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
agentex-ui/components/task-header/investigate-traces-button.tsx:31
**Silent path change from `/beta/monitor` to `/monitor`**

The old URL used `${sgpAppURL}/beta/monitor?...` but the new URL drops the `/beta/` segment. This change is not mentioned in the PR description and may be intentional (the route graduated from beta) or accidental. If SGP still serves the monitor at `/beta/monitor`, this link will silently 404 or redirect to an unrelated page for all users.

`````

</details>

<sub>Reviews (2): Last reviewed commit: ["fix(agentex-ui): use /monitor
instead
of..."](a2dbb4a)
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---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: cdvillegas

agentex-ui/components/task-header/investigate-traces-button.tsx

@@ -5,6 +5,7 @@ import { forwardRef } from 'react';
 import { ArrowRight } from 'lucide-react';
 
 import { useAgentexClient } from '@/components/providers';
+import { useSafeSearchParams } from '@/hooks/use-safe-search-params';
 import { cn } from '@/lib/utils';
 
 type InvestigateTracesButtonProps = {
@@ -18,7 +19,16 @@ export const InvestigateTracesButton = forwardRef<
   InvestigateTracesButtonProps
 >(({ className, disabled = false, traceId, ...props }, ref) => {
   const { sgpAppURL } = useAgentexClient();
-  const sgpTracesURL = `${sgpAppURL}/beta/monitor?trace_id=${traceId}&tt-trace-id=${traceId}`;
+  const { sgpAccountID } = useSafeSearchParams();
+
+  const params = new URLSearchParams({
+    trace_id: traceId,
+    'tt-trace-id': traceId,
+  });
+  if (sgpAccountID) {
+    params.set('account_id', sgpAccountID);
+  }
+  const sgpTracesURL = `${sgpAppURL}/monitor?${params.toString()}`;
 
   if (!sgpAppURL) {
     return null;