decode: show Bind result-column format codes

[sunng87]

Summary

Bind was the only extended-query message with an unrendered field —
result_column_format_codes, the text/binary format the client requested for the
result columns (i.e. the "format of output"). Parse already renders all its
fields.

Change (src/decode.rs)

• New helper format_format_codes(&[i16]) → compact text / binary / [text, binary, …].
• format_bind now appends it: … params: [...] result: <fmt>.

Example

An extended-query session (psql \bind) through the mitm proxy:

[F→B] Parse: <unnamed>  [param types: -]  SELECT $1::int AS n
[F→B] Bind: <unnamed>  <-  <unnamed>  params: [3431]  result: text
[F→B] Execute: <unnamed>
[B→F] DataRow: { n='41' }

Verification

• cargo fmt / clippy -D warnings / test (4/4) clean; nix build and nix flake check pass.
• End-to-end: a psql \bind session decodes with result: text present.

Note

This was originally committed straight onto main; per request it's moved onto this branch for proper review. (Unrelated side note, not changed here: in that trace the param renders as hex 3431 because psql's \bind flags the parameter as binary format — existing behaviour, happy to look at separately.)