Restore validation exit for input-too-large errors

[Copilot]

input-too-large errors for oversized stdin and known-command --initial values were falling back to usage exit 2. The spec requires these validation failures to exit 65 so scripts can distinguish bad CLI usage from oversized input.

• Exit-code mapping

  • Added clet-specific mapping for input-too-large → ExitCodes.ValidationError.
  • Preserved package fallback behavior for other errors.

• Oversized --initial handling

  • Detects oversized --initial for registered aliases before the package parser converts it to usage failure.
  • Unknown aliases still return usage error 2.

• Oversized stdin handling

  • Maps JSON error envelopes with code: input-too-large from md --json back to validation exit 65.

• Regression coverage

  • Updated exit-code unit coverage for clet’s mapper.
  • Updated smoke coverage for oversized md --json stdin.

CommandResult result = new(
    CommandStatus.Error,
    null,
    "input-too-large",
    "...");

CletExitCodes.FromResult(result); // 65

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep oversized unknown aliases as usage errors

When an invocation uses an unknown alias with an oversized --initial and --json, TryHandleOversizedInitial deliberately falls through because registry.TryResolve fails, but the package parser still emits an input-too-large JSON error before command resolution. This broad remap then turns that usage failure into exit 65, contradicting the spec/commit note that unknown aliases must remain usage errors (exit 2), e.g. clet nope --json --initial <65K> now exits as validation instead of unknown-command usage.

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