BUG: planning updates render a literal remaining_steps placeholder and ignore run overrides

[tlysanhuo]

Description

Updated planning prompts do not receive a usable remaining-step budget.

All three built-in prompt templates contain the single-braced text {remaining_steps}. populate_template() uses Jinja with StrictUndefined, so that text is not interpolated and reaches the model verbatim.

There is a second issue on the same data path: _generate_planning_step() derives the value from self.max_steps. When run(..., max_steps=N) overrides the configured default, the execution loop uses N, but planning still uses the constructor value.

Reproduction

from smolagents import CodeAgent
from smolagents.models import ChatMessage, MessageRole, Model

class RecordingModel(Model):
    def __init__(self):
        super().__init__(model_id="recording")
        self.calls = []

    def generate(self, messages, **kwargs):
        self.calls.append(messages)
        return ChatMessage(
            role=MessageRole.ASSISTANT,
            content="<code>\nprint(\"continue\")\n</code>",
        )

model = RecordingModel()
agent = CodeAgent(
    tools=[],
    model=model,
    max_steps=20,
    planning_interval=1,
)
agent.run("demo", max_steps=3)

for messages in model.calls:
    text = str(messages)
    if "steps remaining" in text:
        print(text)

The update-planning requests contain:

Beware that you have {remaining_steps} steps remaining.

After correcting the Jinja syntax alone, step 2 would still be calculated from 20 - 2 rather than the effective per-run budget 3 - 2.

Expected behavior

• The built-in CodeAgent, structured CodeAgent, and ToolCallingAgent planning templates render a numeric remaining-step value.
• The value comes from the effective budget of the current run, including a run(max_steps=...) override.
• Regression tests cover every built-in planning template and the per-run override path.

Environment

• smolagents: 1.27.0.dev0 at e3a5b8994b301983b91c0325546e9dc82eab8cf0
• Python: 3.12.13
• OS: macOS

[ErenAta16]

Confirmed both halves, opened #2526 alongside a related max_steps=0 fix. Verified populate_template really does use Jinja2 with StrictUndefined (single-brace {remaining_steps} is genuinely never interpolated, not just a display quirk), and threaded the effective max_steps through _run_stream -> _generate_planning_step so the value matches what the execution loop actually uses.