[nextjs] [Pages Router] Adjust static path generation when multisite is disabled

[yavorsk]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

When multisite is disabled in sitecore.config MultisiteProxy does not run and does not rewrite paths by adding site segment ( /_site_my-site/...), while the same setting does not affect the behavior of getPagePaths - paths are generated with the site segment.
This PR adjusts getPagePaths to eliminate this discrepancy.

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate) - in Pages Router app explicitly disable multisite; run in prod mode; verify that generated paths are without site prefix, personalize and redirects should work as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[github-actions]

📦 Package Size and Test Coverage Report

Package	Base Size	PR Size	Δ Change	Base Coverage	PR Coverage	Δ Change
analytics-core	122.52 KB	122.52 KB	✅ 0.00 KB	100.00%	100.00%	0.00%
cli	58.51 KB	58.51 KB	✅ 0.00 KB	69.73%	69.73%	0.00%
core	505.17 KB	505.17 KB	✅ 0.00 KB	91.23%	91.23%	0.00%
create-content-sdk-app	240.61 KB	240.66 KB	🔺 +0.05 KB	96.22%	96.22%	0.00%
events	76.92 KB	76.92 KB	✅ 0.00 KB	98.40%	98.40%	0.00%
nextjs	423.95 KB	426.71 KB	🔺 +2.76 KB	89.69%	89.75%	+0.06%
personalize	63.45 KB	63.45 KB	✅ 0.00 KB	99.32%	99.32%	0.00%
react	289.13 KB	289.13 KB	✅ 0.00 KB	93.64%	93.64%	0.00%
search	6.40 KB	6.40 KB	✅ 0.00 KB	98.57%	98.57%	0.00%
Total	—	—	🔺 +2.81 KB	—	—	—