Do not echo rejected connection parameter values

[PingoLee]

A security-sensitive libpq keyword set to a value Postgres.jl cannot honor is rejected with an ArgumentError that printed key=value. For sslpassword that value is the client key passphrase, so it ended up in error messages, logs and stack traces:

Postgres.parse_dsn("host=h sslpassword=hunter2")
# ArgumentError: connection parameter "sslpassword=hunter2" is not supported by Postgres.jl and cannot be safely ignored

The URI form (?sslpassword=...) goes through the same check.

The error now names only the parameter, as the unrecognized-parameter error already does. This follows the rule from #5 that displaying connection options must never reveal a secret (ConnectionParams shows password=***). The value is dropped for every key rather than only sslpassword, so no list of secret keys has to be kept in sync. Which values are accepted or rejected is unchanged. No public API or runtime dependency changes; only the error text differs.

Malformed URIs can still echo credentials through URIs.jl's ParseError, and a mistyped scheme can reach the keyword parser's "missing '='" error. Both have a different cause and are left for a separate issue.

Validation:

• New checks in "Connection String Parsing" cover the keyword and URI forms. With the src change reverted, the two "passphrase not in message" checks fail.
• Pkg.test(): 5347 checks pass on Julia 1.12.7 and 5321 on Julia 1.10.12 (Linux x64).
• Fork CI on d34c93a: tests pass on Linux, Windows and macOS (Julia min, 1 and pre), with Docker integration against PostgreSQL 14, 15, 17 and 18 and SCRAM and MD5 auth. Only the Codecov upload failed, because the fork has no token.

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