Fix ParseSSE function causing Leo to get cut off

[nvonpentz]

Resolves brave/brave-browser#34321

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

This one is hard to reproduce without setting up the AI chat server. For now, I think it is sufficient to just make sure that you can send a chat (with any model) and you do get a response, and it's not cut off.

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.62.67 Chromium: 119.0.6045.163 (Official Build) nightly (64-bit)
-- | --
Revision | 223789bf687abf95feb51f6c9fb2acdfd8c3a359
OS | Windows 11 Version 22H2 (Build 22621.2715)

Basically used the STR/guidance from #21018 (comment) and went through the following:

• ensured that you get a Premium prompt/modal within the sidebar panel when attempting to pick a premium model
• ensured that you can run through the signup flow and retrieve the needed tokens for users with a subscription

Example	Example

llama-2-13b-chat

• ensured that you can ask the llama a question and get an answer that isn't cut off
• ensured that you can summarize a page/ask follow up questions without the answers being cut off

Example	Example	Example	Example	Example

llama-2-70b-chat

• ensured that you can ask the llama a question and get an answer that isn't cut off
• ensured that you can summarize a page/ask follow up questions without the answers being cut off

Example	Example	Example	Example	Example

claude-instant-v1

• ensured that you can ask the claude a question and get an answer that isn't cut off
• ensured that you can summarize a page/ask follow up questions without the answers being cut off

Example	Example	Example	Example	Example

[nullhook]

i'm curious how this behaved/s before and after.

std::move(json.data()) is unusual, doesn't make sense and behaved sporadically odd.

for a moment, let's examine this: json.data() returns a const char*, the consumer of this pointer reads char up to the null terminator string.

how did this end up with sentence cut-offs?

[bbondy]

https://bravesoftware.slack.com/archives/C05NDQP03AQ/p1700249757581809?thread_ts=1699393531.992589&cid=C05NDQP03AQ