refactor(proxy): migrate PostgreSQL protocol to pg-proto

[freshtonic]

Migrate the proxy PostgreSQL protocol implementation to published pg-proto 0.1.0 and remove the superseded local protocol stack.

What changed

• delegate tagged framing, direction-specific decoding, validation, and serialization to pg-proto
• delegate startup, SSL, cancellation, authentication, and diagnostic message representation to pg-proto
• pair downstream/upstream protocol state machines while preserving pipelining
• retain only CipherStash-specific state and message handling for SQL, parameter, metadata, and row rewrites
• preserve buffering, authentication policy, TLS policy, error factories, metrics, timeouts, and cancellation forwarding
• reject malformed, oversized, and unknown-tag frames through the pg-proto codecs

Legacy cleanup

Removed the handwritten frontend/backend tag enums, startup codes/messages, authentication wire model, ReadyForQuery and Terminate serializers, frame length/tag parsing, diagnostic serialization, and redundant Parse/Bind wire counts. The cleanup commit removes 1,251 lines while adding 296 lines of typed integration and retained policy.

Verification

• cargo fmt --all -- --check
• cargo clippy -p cipherstash-proxy --all-targets -- -D warnings
• CS_PROMETHEUS__ENABLED unset: 127/127 unit tests pass, plus doc tests
• focused PostgreSQL protocol and rewrite tests pass
• live MD5 authentication, simple query, extended Parse/Bind/Execute, invalid Bind, and Sync recovery smoke tests passed before the cleanup; the cleanup preserves those typed paths
• git diff --check

Full Docker/TLS/COPY/asynchronous-message integration coverage has not yet been run.

Acknowledgment

By submitting this pull request, I confirm that CipherStash can use, modify, copy, and redistribute this contribution, under the terms of CipherStash's choice.

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