CMM-1095: Fix stats off-by-an-hour by reading the site's named timezone

[adalpari]

Description

Daily stats reset to zero an hour early (e.g. at 23:00 instead of 00:00) for
users whose site uses a DST timezone, such as Europe/Madrid.

The previous fix (#22480) intended to make the stats "day" boundary DST-safe by
storing the site's named (Olson) timezone in SiteModel.timezone and letting
SiteUtils.getNormalizedTimezone resolve DST from it. However, it read the wrong
JSON field: the WP.com API exposes the named timezone under different keys
depending on the endpoint:

• Site info (/me/sites, /sites/$id) → options.timezone (e.g. "Europe/Madrid")
• Site settings (/sites/$id/settings) → settings.timezone_string

Because the site-info response never contains timezone_string, the condition was
always false and SiteModel.timezone fell back to the numeric gmt_offset. A
numeric offset carries no DST rules and goes stale across DST transitions, so
"today" could be computed an hour ahead — at 23:00 Madrid (22:00 UTC → 00:00 in a
stale GMT+2) the app requested tomorrow's date and the server returned zeros.

This PR reads the correct options.timezone field. The DST-safe normalization
added in #22480 is unchanged and now actually receives a named zone.

Note: this affects the legacy stats screen (ui/stats/refresh/…), which derives
the request date from SiteModel.timezone. The newer ui/newstats/… screen uses
the device timezone and is out of scope here.

Testing instructions

Note

This fix only affects the old stats screen. To see the old stats screen you
must log in with a non-admin user (admins are routed to the new stats screen).

1. Log in with a non-admin user on a WordPress.com/Jetpack site.
2. Open Stats and confirm the old stats screen is shown.

• Verify the stats (Traffic / Days, Today) load and show the correct data for
  the current day, matching the web Jetpack stats.

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Android by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Android
	Build Type	Debug
	Version	pr23124-1275aa6
	Build Number	1498
	Application ID	com.jetpack.android.prealpha
	Commit	1275aa6
	Installation URL	5f96ti89llqso

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Android by scanning the QR code below to install the corresponding build.

	App Name	WordPress Android
	Build Type	Debug
	Version	pr23124-1275aa6
	Build Number	1498
	Application ID	org.wordpress.android.prealpha
	Commit	1275aa6
	Installation URL	1g5mrrkag0d3g

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 37.74%. Comparing base (44f24ca) to head (05e1861).

Files with missing lines	Patch %	Lines
...id/fluxc/network/rest/wpcom/site/SiteRestClient.kt	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #23124      +/-   ##
==========================================
+ Coverage   37.71%   37.74%   +0.02%     
==========================================
  Files        2343     2343              
  Lines      127380   127380              
  Branches    17674    17674              
==========================================
+ Hits        48047    48078      +31     
+ Misses      75393    75358      -35     
- Partials     3940     3944       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nbradbury]

Fix is good!