Add step parsing for each agent section

[myanvoos]

This PR adds step parsing for each agent section to transform the raw content at each step into a more human-readable format, with collapsible subsections containing a text preview of the chat prompt/response (first 50 words).

Preview

[myanvoos]

/gcbrun exp -n mv -m vertex_ai_gemini-2-flash-chat -ag

[DonggeLiu]

Thanks, this looks great! :)

Here are a few thoughts on the UX:

1. The downward-pointing angle for all steps is a bit ambiguous. Just to confirm my understanding: are steps 1-2 collapsed, step 3 expanded (but its content hidden), and step 4 fully expanded?

2. The titles would be much more informative if they showed the tool being used, like 'Step 1 - Bash' or 'Step 5 - Conclusion'. This would make the agent's logic much easier to scan. It'd be great if this could handle multiple tools (optional), too (e.g., 'Step 5 - Bash, Conclusion').

3. Building on that, what do you think about simplifying the UI by having only one layer of collapse? A collapsed step would show just the new informative title, and an expanded one would reveal all its content, like your 'Step 4' example.

These are just my initial impressions: Happy to hear what you think!

[myanvoos]

1. Yep, your understanding is right.
2. Good point, I'll add that in. My current plan is to parse the unique tags and put that in the title i.e. if there's at least one <bash> line and at least one <reason> line, the title will show 'Step X - Bash, Reasoning`.
3. With the new informative title, I think this is a good idea.
4. What do you think about putting some kind of content preview in the title so that users can see at a glance the difference between, say, two steps that both have 'Bash, Reasoning'?

So instead of

Step 1 - Bash, Reasoning
Step 2 - Bash, Reasoning
Step 3 - Bash, Conclusion

We use some variant of

Step 1 - Bash ('janet_fiber'), Reasoning, Bash ('typedef struct JanetFunction'), Reasoning, Bash ('JanetFunction')
Step 2 - Bash ('typedef struct JanetFunction'), Bash ('JanetFunction'), Reasoning, Bash ('janet_wrap_cfunction')
Step 3 - Bash ('janet_wrap_cfunction'), Conclusion

to represent the following logs:

Step 1:
<bash>
grep -rn 'janet_fiber(' /src/janet/
</bash>
...
<reason>
Find the definition of the `JanetFunction` type.
</reason>
<bash>
grep -rn 'typedef struct JanetFunction' /src/janet/
</bash>

<reason>
Examine how to create a JanetFunction.  Looking for examples of its usage.
</reason>
<bash>
grep -rn 'JanetFunction' /src/janet/ | grep -v 'typedef struct JanetFunction'
</bash>

Step 2:
<bash>
grep -rn 'typedef struct JanetFunction' /src/janet/
</bash>
...
<bash>
grep -rn 'JanetFunction' /src/janet/ | grep -v 'typedef struct JanetFunction'
</bash>
...
<reason>
It appears that creating a `JanetFunction` directly is complex.  Instead, I'll try to get a `JanetFunction` by creating a Janet function using `janet_wrap_cfunction`.
</reason>
<bash>
grep -rn 'janet_wrap_cfunction' /src/janet/
</bash>
...

Step 3:
<bash>
grep -rn 'janet_wrap_cfunction' /src/janet/
</bash>
<conclusion>
The fuzz target needs to initialize Janet, create a dummy C function, wrap it as a `Janet` value using `janet_wrap_cfunction`, create a `JanetFiber` with this value, call `janet_loop_fiber` with the fiber, and then deinitialize Janet. The build script doesn't need modification.
</conclusion>

I think this can actually be made toggle-able, so that the user can decide whether they just want a high-level skim of the tools used or they want a more low-level preview of the collapsed content.