Allow to stop a streamed Leo response

[bbondy]

Currently if you have a question you need to wait for the whole answer.
Sometimes you notice right away after submitting that you forgot to specify something important though and you want to stop the reply early.
The send button should probably change to a stop button for users that want to interrupt a response.

cc @aguscruiz @nullhook @petemill

[aguscruiz]

Added designs here:
https://www.figma.com/design/m0Gdbf0wtqyfEFGm32VLLc/Leo?node-id=3496%3A13366&t=CTUxdd31WfqlpazL-1

I also have the iOS version, can you create an issue for that one?

[MadhaviSeelam]

Verification PASSED using

Brave | 1.76.55 Chromium: 133.0.6943.54 (Official Build) beta (64-bit)
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Revision | 8b59c06a32ec3a53b2caada1e6c43c3e5c35d64f
OS | Windows 11 Version 24H2 (Build 26100.2894)

1. Installed 1.76.55
2. launched Brave
3. load a page https://support.brave.com/hc/en-us/articles/26727364100493-What-are-the-differences-between-Leo-s-AI-Models
4. clicked Leo AI icon in the sidebar
5. clicked Summarize this page
6. confirmed Stop button is shown
7. clicked Stop button
8. confirmed Leo response was stopped as expected

2025-02-12_16h22_01.mp4

[hffvld]

Verified on Pixel 7 using version(s):

Device/OS: Pixel 7 / panther_beta-user 16 BP22.250103.008 release-keys
Brave build: 1.76.58 
Chromium: 133.0.6943.98 (Official Build) beta (64-bit) 

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STEPS:

1. Launch Brave
2. Go to https://support.brave.com/hc/en-us/articles/26727364100493-What-are-the-differences-between-Leo-s-AI-Models
3. Open Leo
4. Tap Summarize this page > Verify that Stop button is shown
5. Tap the Stop button > Verify that Leo's response was stopped as expected

ACTUAL RESULTS:

• Verified that the Stop button is shown when Leo runs the query response.
• Verified that Leo's response has stopped as expected when tapping the Stop button.

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