fix: clamp maxAllowedPromptTokens to zero to prevent silent autocomplete failures

[rekha0suthar]

Problem

When using Ollama with a num_ctx value equal to (or smaller than) Continue's default maxTokens, autocomplete silently produces no output. No error is shown, no completion appears — it just does nothing.

Root Cause

In renderPromptWithTokenLimit, the pruneLength helper calculates how many tokens to remove from the prompt:

function pruneLength(llm: ILLM, prompt: string): number {
  const contextLength = llm.contextLength;
  const reservedTokens = llm.completionOptions.maxTokens ?? DEFAULT_MAX_TOKENS;
  const safetyBuffer = getTokenCountingBufferSafety(contextLength);
  const maxAllowedPromptTokens = contextLength - reservedTokens - safetyBuffer;
  const promptTokenCount = countTokens(prompt, llm.model);
  return promptTokenCount - maxAllowedPromptTokens;
}

When contextLength ≤ reservedTokens + safetyBuffer (which happens when Ollama's num_ctx equals Continue's default maxTokens), maxAllowedPromptTokens becomes negative. This causes pruneLength to return promptTokenCount - (negative), i.e. a very large positive number. The pruner then tries to drop more tokens than the entire prompt contains, reducing it to nothing.

Fix

Clamp maxAllowedPromptTokens to a minimum of 0:

const maxAllowedPromptTokens = Math.max(
  0,
  contextLength - reservedTokens - safetyBuffer,
);

This ensures the pruner never requests removal of more tokens than the prompt has, preventing the silent empty-output failure.

Reproducing

1. Set Ollama num_ctx to the same value as Continue's default maxTokens (600)
2. Open a file and wait for autocomplete to trigger
3. Before: no suggestion appears, no error in logs
4. After: autocomplete works correctly, prompt is used as-is (no pruning needed)

Fixes #13038

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