Your question seems restricted only to the lines:

swap_rate = float(irs.rate(solver=solver))
print(swap_rate)
rolled_swap_rate = float(irs.rate(ESTR.roll("3m")))

with the comment that you are not passing a Solver. That's OK, you are passing a Curve. Python is actually interpreting this (via positional arguments) as equivalent to:

rolled_swap_rate = float(irs.rate(curves=ESTR.roll("3m")))

The roll method returns a new Curve so this is all valid.

The part of docs that explains this is here

Btw you should add the "tgt" to your ESTR curve definintion.

Your question seems restricted only to the lines:

swap_rate = float(irs.rate(solver=solver))
print(swap_rate)
rolled_swap_rate = float(irs.rate(ESTR.roll("3m")))

with the comment that you are not passing a Solver. That's OK, you are passing a Curve. Python is actually interpreting this (via positional arguments) as equivalent to:

rolled_swap_rate = float(irs.rate(curves=ESTR.roll("3m")))

The roll method returns a new Curve so this is all valid.

The part of docs that explains this is here

Btw you should add the "tgt" calendar to your ESTR curve definintion.