isotp: extend long reassembly buffers in place

[KernelClint]

The opt-in software ISO-TP socket reassembles fragmented application messages carried over CAN. A
32-bit First Frame length starts an attacker-sized message at
scapy/contrib/isotp/isotp_soft_socket.py:986-1005,
and every valid Consecutive Frame copies the growing immutable receive buffer at
scapy/contrib/isotp/isotp_soft_socket.py:1061-1070.

From 131,072 to 1,048,576 application bytes, median receive processing grew from 32.10 ms to
1,206.87 ms, with a 2.06 exponent and a 2.3% noise floor. Patched times were 7.44 ms to 59.91 ms,
with a 0.99 exponent and a 6.3% noise floor.

This change uses a bytearray only for extended-length messages, extends it in place, and converts
once on completion. The focused regression failed on the unmodified revision and passed with the
patch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.60%. Comparing base (ba8641a) to head (492e6bd).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5109   +/-   ##
=======================================
  Coverage   80.59%   80.60%           
=======================================
  Files         390      390           
  Lines       96892    96895    +3     
=======================================
+ Hits        78094    78098    +4     
+ Misses      18798    18797    -1     

Files with missing lines	Coverage Δ
scapy/contrib/isotp/isotp_soft_socket.py	84.96% <100.00%> (+0.13%)	⬆️

... and 11 files with indirect coverage changes

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[KernelClint]

Yes — you're right, and I've changed it in 492e6bd0.

I'd added the length check to keep the short-message path untouched, but measuring it, that isn't
what happens. Reassembling one normal two-frame message:

	ns/message, three runs
unmodified	710, 720, 700
conditional	777, 729, 756
always bytearray	800, 748, 748

The two variants are indistinguishable — the spread between repeated runs of the same build is
wider than the gap between builds. So the condition bought nothing and cost three isinstance
branches and a union type.

Always using a bytearray removes all of that: rx_buf is Optional[bytearray] throughout and
converts to bytes once, where it is sent to the queue. test/contrib/isotp_soft_socket.uts
passes.