fix(observability): keep usage chart labels visible

[kkkhs]

Summary

Fixes #5968.

Usage charts were using a negative left margin, which pushed wider y-axis labels outside the SVG clip. This keeps the left chart margin at zero so labels like 20Kms and $4.00 stay visible.

Testing

Verified locally

• pnpm --filter @agenta/oss exec vitest run src/components/pages/observability/dashboard/CustomAreaChart.test.tsx
• pnpm exec eslint oss/src/components/pages/observability/dashboard/CustomAreaChart.tsx oss/src/components/pages/observability/dashboard/CustomAreaChart.test.tsx
• pnpm --filter @agenta/oss exec tsc --noEmit
• git diff --check

Added or updated tests

Added a regression test for the chart margin used by the observability usage charts.

QA follow-up

Verify the expanded Usage card with latency and cost values wide enough to produce labels such as 20Kms or $4.00.

Demo

Draft PR: demo recording pending before ready-for-review.

Checklist

• I have included a video or screen recording for UI changes, or marked Demo as N/A
• Relevant tests pass locally
• Relevant linting and formatting pass locally
• I have signed the CLA, or I will sign it when the bot prompts me

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• Testing
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• Slack support

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[ashrafchowdury]

Hey @kkkhs, thank you for the fix. Can you please sign the CLA license? After that, we can merge the PR