(feat): Support PostgreSQL protocol 3.2 negotiation for frontend clients

[sarajmunjal]

This PR adds frontend-side PostgreSQL protocol 3.2 support to PgDog - addressing #578.

PgDog now accepts 3.2 startup requests, negotiates newer 3.x minor versions down with NegotiateProtocolVersion, reports unsupported pq.* startup options in that response, and returns cancel keys / BackendKeyData in the right shape for the negotiated protocol. Backend connections to PostgreSQL are unchanged.

What changed

• added protocol version parsing/negotiation and NegotiateProtocolVersion
• updated startup parsing for 3.2, 3.3+ -> 3.2, and pq.* option reporting
• updated BackendKeyData / CancelRequest handling for protocol-dependent secret lengths
• generated client cancel keys based on the negotiated frontend protocol version
• added unit and integration coverage for negotiation and extended cancel-key behavior
• added an independent psql 18 smoke test pinned to exact 3.0 and 3.2

Compatibility

3.0 clients continue to work
3.2 clients are accepted directly
newer 3.x minor versions negotiate down to 3.2
non-3.x major versions remain unsupported

Verification

Ran locally:

cargo fmt --all
cargo test -p pgdog --lib --no-run
cargo test -p rust --test mod --no-run
cargo test -p rust --test mod integration::protocol_version:: -- --nocapture
bash integration/complex/protocol_version/run.sh

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 86.85015% with 43 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/listener.rs	0.00%	16 Missing ⚠️
pgdog/src/net/messages/backend_key.rs	89.47%	10 Missing ⚠️
pgdog/src/net/messages/protocol_version.rs	80.85%	9 Missing ⚠️
pgdog/src/net/messages/hello.rs	97.05%	3 Missing ⚠️
pgdog/src/net/messages/mod.rs	0.00%	3 Missing ⚠️
pgdog/src/backend/server.rs	0.00%	1 Missing ⚠️
...dog/src/net/messages/negotiate_protocol_version.rs	97.56%	1 Missing ⚠️

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