feat(ai-overview): Cost display improvements/caveats (#4735)

## Description

<!-- What changed? Include screenshots if you modify the UI. -->

- Estimated Cost tile always shows a tooltip explaining cost is based on
uncached token usage (previously only shown when some calls were
unpriced).
<img width="420" height="222" alt="image"
src="https://github.com/user-attachments/assets/0e9cc2a1-a388-48bc-89f2-3e768d658e4c"
/>

- Renamed cost charts to "Estimated cost ..." / "Estimated cost by ..."
across both AI dashboards.
<img width="1493" height="649" alt="image"
src="https://github.com/user-attachments/assets/3f9e9099-724b-49df-b45c-de402744f7c4"
/>
- Added tooltip icons to every chart in the Cost section explaining how
cost is calculated.
<img width="369" height="118" alt="image"
src="https://github.com/user-attachments/assets/ccb5abd2-8564-4aea-8038-d3218db9d851"
/>
- Cost values under $0.01 now show as <$0.01 on bar chart axes and
labels (line charts/tooltips keep full precision) (previously it would
overflow).
<img width="167" height="230" alt="image"
src="https://github.com/user-attachments/assets/71d60e92-5810-4f7f-aaa1-193b40bb1968"
/>
- Fixed the "Open in Insights" menu item's hover highlight to align
flush with the menu border, and removed its stray focus outline.
<img width="211" height="132" alt="image"
src="https://github.com/user-attachments/assets/d0249205-120f-4752-92ba-7ba0bf1b2a71"
/>


## Release note

<!-- User-facing release note. Use "None" if this does not need release
notes. -->
None.

## Migration note

<!-- Upgrade, deployment, config, schema, or compatibility notes. Use
"None" if none. -->
None.

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