fix(deepseek): don't drop streamed content deltas equal to "0"

[moaines]

Problem

Numbers ending in 0 are corrupted when streamed through the DeepSeek provider.
The provider emits 3800 as two content deltas — "380" then "0" — and the
stream handler skips any delta that is exactly the string "0", so the assembled
text is 380. Same for 2200 → 220.

A raw API call (and Prism's non-streaming asText()) return the value correctly.
The corruption is introduced by Prism's stream parsing, not by the model.

Root cause

src/Providers/DeepSeek/Handlers/Stream.php:

$content = $this->extractContentDelta($data);
if ($content !== '' && $content !== '0') {
    // yield TextDeltaEvent ...
    continue;
}
// a "0" delta falls through and is treated as "no content"

A "0" delta is legitimate content and must be yielded. The same guard exists
for reasoning deltas in the DeepSeek handler and for thinking deltas in the XAI
handler.

Changes

• src/Providers/DeepSeek/Handlers/Stream.php — yield "0" content and reasoning deltas.
• src/Providers/XAI/Handlers/Stream.php — yield "0" thinking deltas.
• tests/Providers/DeepSeek/StreamTest.php, tests/Providers/XAI/StreamTest.php — regression tests
  (fail without the fix, pass with it), plus SSE fixtures.

Notes

• The existing XAI test does not truncate 0 mid stream covers the same bug for
  XAI content deltas (already fixed in that path); this PR completes the same fix
  for DeepSeek content + reasoning and XAI thinking deltas.
• The !== '0' guard also appears in several MessageMap classes (Anthropic,
  OpenAI, Gemini, OpenRouter) — that is outgoing message serialization, a
  different code path, and is left out of this PR.

Fixes #1034