Optimize unbounded byte scans with memchr (#26265)

## Summary

This PR adds `memchr` for some low-hanging performance improvements
(namely, in MCP stdio, Ollama streaming, and full message-history
newline counts).

Codex produced the following release benchmarks:

| Operation | Before | After | Speedup |
| --- | ---: | ---: | ---: |
| MCP 1 MiB chunked line | 2.172 s | 3.984 ms | 545x |
| Ollama 1 MiB chunked line | 1.673 s | 2.790 ms | 600x |
| Count newlines in 10 MiB history | 132.83 ms | 20.05 ms | 6.6x |

With a "real" MCP setup (`ExecutorStdioServerLauncher` started a Python
MCP server, completed `initialize`, requested `tools/list`, and
deserialized a 1 MiB tool description over newline-delimited stdio),
it's about 16x faster end-to-end:

| Branch | 50 calls | Per call |
| --- | ---: | ---: |
| `main` | 862.53 ms | 17.25 ms |
| this branch | 53.89 ms | 1.08 ms |

`memchr` is already in our dependency tree and extremely widely used for
this kind of optimized scanning.

codex-rs/Cargo.lock

@@ -3293,6 +3293,7 @@ name = "codex-message-history"
 version = "0.0.0"
 dependencies = [
  "codex-config",
+ "memchr",
  "pretty_assertions",
  "serde",
  "serde_json",
@@ -3407,6 +3408,7 @@ dependencies = [
  "codex-core",
  "codex-model-provider-info",
  "futures",
+ "memchr",
  "pretty_assertions",
  "reqwest 0.12.28",
  "semver",
@@ -3576,6 +3578,7 @@ dependencies = [
  "codex-utils-pty",
  "futures",
  "keyring",
+ "memchr",
  "oauth2",
  "pretty_assertions",
  "reqwest 0.13.4",