i18n(ja): add missing ni particle for clarity in diagnostics N-fold wording

[yahonda]

What is changed, added or deleted? (Required)

In dashboard/dashboard-diagnostics-usage.md, two "N-fold" sentences (QPS decreased 0.93x, P999 query latency increased 1.54x) lacked the に particle that sibling sentences in the same bullet list already use (e.g. 2.48 倍に増加しました, 7.61 倍に増加しました). This PR adds に to both sentences for consistency and to make the ratio reading ("became N times the original value") explicit.

Verified against the actual numbers in the referenced screenshot (/media/dashboard/dashboard-diagnostics-usage2.png): tidb_qps shows 34927.00 / 37658.00 ≈ 0.93, confirming this is a straightforward new/old ratio, not a subtractive change.

Which TiDB version(s) do your changes apply to? (Required)

• i18n-ja-release-8.5 (TiDB Japanese documentation for TiDB 8.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• Documentation
  • Improved the Japanese wording of diagnostic report metric descriptions for clarity and grammatical correctness.

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ℹ️ Recent review info

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📥 Commits

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• dashboard/dashboard-diagnostics-usage.md

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📝 Walkthrough

Walkthrough

The change revises two Japanese diagnostic metric descriptions. It inserts the particle "に" after "倍" without changing the documented metric meaning or behavior.

Changes

Diagnostic documentation wording

Layer / File(s)	Summary
Metric description wording dashboard/dashboard-diagnostics-usage.md	The tidb_qps and tidb_query_duration descriptions use revised Japanese grammar.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to e2e99

This documentation-only wording update clarifies two Japanese metric descriptions without changing product behavior or metric interpretation. It is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description explains the two wording changes, identifies the affected file and version branch, documents validation, and includes AI involvement and scope checklists. It is complete enough for rev…
Title check	✅ Passed	The title clearly identifies the Japanese localization change and the missing particle in the diagnostics N-fold wording.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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