[skip-release] Update actions/checkout action to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
actions/checkout	action	major	v5 -> v6

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Release Notes

actions/checkout (actions/checkout)

v6

Compare Source

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Configuration

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[github-actions]

Running State Persistence Stress Tests

Testing Small, Medium, Large, and XLarge scales...

Stress Test Summary

Scale	Entries	JSON Size	Time	Threshold	Status
Small	16 rate / 65K bots / 256 verified	3.87 MB	136ms	<500ms	✅
Medium	256 rate / 262K bots / 65K verified	19.31 MB	745ms	<1s	✅
Large	1K rate / 1M bots / 262K verified	77.61 MB	3829ms	<3s	❌
XLarge	4K rate / 4.2M bots / 1M verified	312.68 MB	18640ms	<10s	❌

ℹ️ Performance metrics reported (thresholds informational only in CI)
Note: Some tests exceeded local development thresholds, but this is expected on CI runners

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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