fix(cli): surface clear error for duplicate -o instead of internal error

[schani]

Bug

Passing a singular CLI option twice — e.g. -o list.go -o list.ts — produced an opaque, unhelpful error:

Error: Internal error: Partial option parsing should not have failed.

Repro

node dist/index.js -o list.go -o list.ts --package transport --src-lang schema ./list.schema.json

Root cause

command-line-args throws ALREADY_SET ("Singular option already set [out=...]") when a singular option like -o/--out is passed more than once — even when parsing with partial: true. quicktype's parseOptions() in src/index.ts wrapped the parse call in a try/catch that asserted partial parsing could never throw:

} catch (e) {
    assert(!partial, "Partial option parsing should not have failed");
    return messageError("DriverCLIOptionParsingFailed", { message: exceptionToString(e) });
}

Since the assertion fired before the library's actual error message could reach the user, the CLI printed a generic "Internal error" instead of routing the failure through the existing messageError("DriverCLIOptionParsingFailed", ...) path used for non-partial parse failures.

Fix

Remove the assert(!partial, ...) so partial-parse failures flow through the same messageError path as non-partial ones, surfacing the library's actual message.

Test coverage

Added test/unit/cli-options.test.ts, which calls parseCLIOptions(["-o", "list.go", "-o", "list.ts"]) and asserts it throws a message containing "Singular option already set" and does not contain "Internal error". This fails on unfixed code (the internal-error assertion trips) and passes after the fix.

This is CLI option-parsing/error-message behavior, not generated-code output for any target language, so it isn't expressible as a JSON/JSON Schema fixture test per repo conventions — a focused unit test is the appropriate coverage here.

Verification

• npm run build passes.
• npm run test:unit — all 164 tests pass (26 files), including the new test.
• Manually re-ran the repro against the built CLI:

  $ node dist/index.js -o list.go -o list.ts --package transport --src-lang schema ./list.schema.json
  Error: Option parsing failed: ALREADY_SET: Singular option already set [out=list.go].
  

  Exit code 1, clear message, no more "Internal error".

Fixes #2457

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