fix(planners): keep all leading parallel function calls

[nyxst4ck]

Link to Issue or Description of Change

No existing issue — describing the bug here.

Problem:

PlanReActPlanner.process_planning_response drops every parallel function call
except the first when the model's response starts with a function call.

The trailing-group collector is guarded by:

first_fc_part_index = -1
for i in range(len(response_parts)):
    if response_parts[i].function_call:
        ...
        first_fc_part_index = i
        break
    ...
if first_fc_part_index > 0:   # <-- bug
    j = first_fc_part_index + 1
    while j < len(response_parts):
        ...

first_fc_part_index is the index of the first function call (sentinel -1).
When the first part is a function call its index is 0, so > 0 is false and
the loop that collects the rest of the parallel call group never runs — the
first call is kept, the rest are silently dropped. Responses that begin with
text (index >= 1) work, which is why this wasn't noticed. Gemini emitting a
group of parallel function calls as the first parts of a turn is a realistic
case (and is what the planner instruction encourages).

Solution:

Change the guard to >= 0 so a leading function call is handled the same as one
preceded by text.

Testing Plan

Unit Tests:

• Added tests/unittests/planners/test_plan_re_act_planner.py.
• All unit tests pass locally.

test_preserves_all_leading_parallel_function_calls is red on main
(returns only ["get_weather"]) and green with this change (returns
["get_weather", "get_time"]). A companion test confirms the leading-text case
still works.

$ pytest tests/unittests/planners/test_plan_re_act_planner.py -q
2 passed
$ pytest tests/unittests/flows/llm_flows/test_nl_planning.py -q
7 passed

pyink + isort clean.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.