sessions: drop the reassembly minimum when the buffer start moves

[KernelClint]

A complete LDAP bind is dropped if its first segment arrives last. No error, no Raw layer — it
never reaches the caller.

#5101 records the declared message length (scapy/layers/ldap.py:1111) so TCPSession skips
re-parsing until the buffer is that long (scapy/sessions.py:392). The count starts at the
buffer's beginning. A segment belonging before everything received so far makes StringBuffer
prepend and move that beginning (scapy/sessions.py:144), so the count points nowhere. Nothing
clears it, and the session waits for a length it can never reach.

noff already tracks the start, so no new state is needed:

+            noff = data.noff
             data.append(new_data, seq)
+            if data.noff != noff:
+                metadata.pop("tcp_min_len", None)

Clearing the count unconditionally would restore the quadratic cost #5101 removed, so we clear it
only when the start actually moved. In-order traffic is unaffected. The buffer at
scapy/sessions.py:312 appends without a sequence number, so its start moves once, before any
count exists.

Test in ldap.uts: a bind whose first four bytes arrive last.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.33%. Comparing base (b38b2e3) to head (a78e07a).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/sessions.py	0.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (b38b2e3) and HEAD (a78e07a). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (b38b2e3)	HEAD (a78e07a)
	11	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5182       +/-   ##
===========================================
- Coverage   80.35%   47.33%   -33.03%     
===========================================
  Files         374      375        +1     
  Lines       97578    97609       +31     
===========================================
- Hits        78411    46204    -32207     
- Misses      19167    51405    +32238     

Files with missing lines	Coverage Δ
scapy/sessions.py	17.89% <0.00%> (-69.61%)	⬇️

... and 279 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.