fix(s7commplus): reconnect around symbolic reads so browse() works on RST-happy firmware

[tommasofaedo]

Follow-up to the TLS validation in #793, on top of #775 (fix-tls-layering-v2).

Problem

browse() runs an EXPLORE (list_datablocks) → a symbolic GetMultiVariables read (_read_typeinfo_rid) → another EXPLORE (the type-info container). On firmware that sends a TCP RST after the first symbolic read per connection — e.g. S7-1200 FW V4.1, the behaviour already documented in read_symbolic()'s own docstring ("TCP RST after first I/Q/M read… reconnect before each I/Q/M read") — the final EXPLORE runs on a dead socket and browse() blows up:

S7ConnectionError('Not connected')
  at _explore_type_info_container -> send_request -> _tls_flush_outgoing -> _iso_conn.send_data

Fix

A lazy reconnect-and-retry:

• connect() stores its arguments; a new _open_connection() / _reconnect() rebuild the session with them.
• _with_reconnect(op) runs op and, only if the PLC dropped the socket (S7ConnectionError), reconnects once and retries. Well-behaved firmware never triggers the retry (the first call succeeds), so there is no behaviour change there.
• browse() wraps the per-DB reads and the container EXPLORE with it; _read_typeinfo_rid now propagates S7ConnectionError (other errors still mean "skip DB").

Also: downgrade the misleading "CreateObject returned error … PLC may require TLS" warning to debug when TLS is already active — this PLC returns a non-zero CreateObject value on a fully functional TLS session, so the warning is noise.

Validation

Live on an S7-1200 FW V4.1 over TLS (secure PG/PC comms). Before: browse() raised Not connected. After:

IArea.input_1    BOOL    (50.A)
QArea.output_1   BOOL    (51.9)
MArea.merker_1   BOOL    (52.9)
MArea.mtag_byte  BYTE    %MB100  (52.B)
MArea.mtag_word  WORD    %MW102  (52.D)
MArea.mtag_dword DWORD   %MD104  (52.E)
MArea.mtag_bool  BOOL    (52.10)

Types and addresses cross-checked against TIA Portal.

• pytest tests/test_client.py tests/test_tag_browser.py tests/test_client_e2e.py → 107 passed, 48 skipped
• ruff check + ruff format clean
• no new mypy errors from the change
• Only validated on FW V4.1 (I don't have other RST-happy firmware to hand); the retry is a no-op on firmware that keeps the socket open.

Refs #793, #775