#30292)

## **Description**

## Problem
Chart data took ~9-10 seconds to display after navigating to token
details. Performance logs showed it takes approximately 9.4 seconds from
WebSocket subscription until the first bar update arrives from the
server.

## Solution
Implemented immediate REST API polling to provide data while waiting for
the first WebSocket update:

1. **Reduced staleness threshold** from 10s to 5s
2. **Added immediate data fetch** via REST API right after WebSocket
subscription

## Timeline Observed
```
T=0ms     - Subscribe to WebSocket
T=~300ms  - First REST data arrives (immediate!)
T=5s      - Second REST poll (WebSocket not ready yet)
T=9.4s    - First WebSocket update arrives, REST polling stops
T=14.4s+  - WebSocket continues with regular updates (~5s intervals)
```

## **Changelog**

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CHANGELOG entry: OHLCV chart data now loads in ~300ms via REST API while
establishing WebSocket connection, reducing initial load time from 9.4s
to 300ms.

## **Related issues**

Fixes:

## **Manual testing steps**

```gherkin
Feature: my feature name

  Scenario: user [verb for user action]
    Given [describe expected initial app state]

    When user [verb for user action]
    Then [describe expected outcome]
```

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

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Every checklist item must be consciously assessed before marking this PR
as
"Ready for review". A checked box means you deliberately considered that
responsibility, not that you literally performed every action listed.

Unchecked boxes are ambiguous: they are not an implicit "N/A" and they
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a silent "skip". See `docs/readme/ready-for-review.md` for the full
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- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I've included tests if applicable
- [ ] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

#### Performance checks (if applicable)

- [ ] I've tested on Android
  - Ideally on a mid-range device; emulator is acceptable
- [ ] I've tested with a power user scenario
- Use these [power-user
SRPs](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/edit-v2/401401446401?draftShareId=9d77e1e1-4bdc-4be1-9ebb-ccd916988d93)
to import wallets with many accounts and tokens
- [ ] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
Guide](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/400085549067/Performance+Guide+for+Engineers).

## **Pre-merge reviewer checklist**

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Reviewer checklist items follow the same semantics as the author
checklist: an
unchecked box is ambiguous, a checked box means the reviewer consciously
assessed that responsibility. See `docs/readme/ready-for-review.md`.
-->

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes chart data fetching timing by triggering immediate REST polls
and a shorter staleness threshold, which could increase network traffic
or cause unexpected polling behavior if timing assumptions are wrong.
> 
> **Overview**
> Reduces perceived OHLCV chart load time by **polling the `/latest`
REST endpoint immediately after a successful WebSocket subscribe**,
instead of waiting for the first WS bar.
> 
> Shortens the WS staleness threshold from `10s` to `5s`, tweaks stale
detection to use `>=`, and adds basic numeric validation in
`fetchLatestBar` to drop malformed REST responses. Tests are updated to
reflect the new timing and the additional initial REST fetch (many cases
now expect **two** `fetch` calls: immediate + staleness/chain-down).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
a3d5c11. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->