fix(server): harden COTP connection framing

[gijzelaerr]

Summary

• echo calling and called TSAP parameters in COTP Connection Confirm frames for Snap7-derived clients
• start a fresh receive deadline after each complete TPKT header
• treat payload timeouts as connection errors to prevent stream desynchronisation
• add exact wire-format and deadline regression coverage

Verification

• uv run pytest — 2076 passed, 82 skipped
• uv run pre-commit run --all-files
• uv build

Closes #889
Closes #890

[gijzelaerr]

@esternu Could you rerun the two pure-Python compatibility reproducers from #889 and #890 against commit c998860? This PR changes the COTP connection-confirmation framing and fragmented/coalesced request handling specifically around those failures. No physical PLC is needed; a pass or remaining traceback from both reproducers would close the validation loop.