core: raise token budget message limits (#29970)

## Why

Token-budget reminder and guidance messages can require more than 1,000
bytes to provide useful model-facing instructions. At the same time,
these strings are injected into model-visible context, so their size
must remain tightly bounded in response to the P0 context-growth
concern. A 2,000-byte runtime cap provides additional room without
allowing the substantially larger context growth of a 4 KiB limit.

## What changed

- raises the runtime byte limits for token-budget reminder templates and
guidance messages from 1,000 to 2,000
- raises the corresponding JSON Schema `maxLength` values to 2,000
- regenerates `codex-rs/core/config.schema.json`

## Testing

- `just test -p codex-features`
- `just test -p codex-core load_config_resolves_token_budget_config
load_config_rejects_invalid_token_budget_reminder_template`

The full `codex-core` test run completed 2,858 tests successfully and
encountered seven unrelated environment-sensitive failures involving
Seatbelt/network environment assertions, MCP capability setup, and abort
timing.

codex-rs/core/src/config/mod.rs

@@ -1088,8 +1088,8 @@ pub(crate) const DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE: &str = concat!(
     "Your context window is nearly exhausted (only {n_remaining} tokens remaining) and will be automatically reset for you soon. ",
     "Once reset, message items in current context window will be cleared in the new window, but notes and history items will be persistent across windows."
 );
-const TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE_MAX_BYTES: usize = 1000;
-const TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES: usize = 1000;
+const TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE_MAX_BYTES: usize = 2000;
+const TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES: usize = 2000;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct TokenBudgetConfig {