Remove chDB-owned clickhouse-connect backend and make clickhouse-connect the single owner

[ShawnChen-Sirius]

Background

chDB added its own clickhouse-connect compatibility backend in chdb.cc_backend,
registered through the clickhouse_connect.backends entry point and shipped via
the chdb[clickhouse-connect] extra.

That made sense when clickhouse-connect only provided the pluggable backend
protocol. However, clickhouse-connect 1.6.0 now ships an experimental in-process
chDB backend directly: get_client(interface='chdb') / chdb:// returns a
standard clickhouse-connect client backed by embedded chDB.

This means there are now two competing implementations of the same integration
surface:

• chDB repo: chdb/cc_backend.py, chdb/cc_extension.py,
  tests/clickhouse_connect/*, clickhouse_connect.backends entry point
• clickhouse-connect repo: native chDB backend maintained alongside the shared
  client/backend core

Proposal

Remove the clickhouse-connect compatibility backend from the chDB repository and
make clickhouse-connect the single owner of the chDB backend implementation.

Concretely, chDB should eventually remove:

• chdb.cc_backend
• chdb.cc_extension, if no longer needed by the clickhouse-connect-owned backend
• the clickhouse-connect optional extra
• the clickhouse_connect.backends entry point
• chDB-side clickhouse-connect backend tests and CI workflow

Rationale

Keeping both implementations creates version skew and ambiguous ownership:

• bug fixes may need to be duplicated across repositories
• public behavior can diverge depending on which backend path users install
• compatibility with clickhouse-connect internals is best maintained in
  clickhouse-connect itself
• chDB can keep exposing the core embedded engine API while clickhouse-connect
  owns client-level parity

Migration target

The supported user path should become:

import clickhouse_connect

client = clickhouse_connect.get_client(interface="chdb")
# or
client = clickhouse_connect.get_client("chdb://memory")

with installation handled by clickhouse-connect, e.g.:

pip install "clickhouse-connect[chdb]"

References

• chDB v4.2.1 shipped the chDB-owned clickhouse-connect backend in Add clickhouse-connect backend #593.
• clickhouse-connect v1.6.0 added the in-process chDB backend in #872.
• This supersedes the earlier split-ownership design discussed in
  Discussion: pluggable Backend protocol for chDB integration (alternative to #753) ClickHouse/clickhouse-connect#809.