feat(kernel): add token remaining/usage-ratio methods to BudgetStatus

[SH20RAJ]

Summary

Fixes #1475.

BudgetStatus in crates/mofa-kernel/src/budget.rs exposes three
convenience methods for cost tracking but nothing equivalent for
token tracking, forcing callers to write manual calculations:

// callers had to write:
let remaining = status.config.max_tokens_per_session
    .map(|max| max.saturating_sub(status.session_tokens));

New API

This PR adds four methods that complete the symmetry between the cost
and token dimensions:

impl BudgetStatus {
    /// Tokens remaining in the current session (clamped to 0).
    pub fn remaining_session_tokens(&self) -> Option<u64>

    /// Tokens remaining in today's budget (clamped to 0).
    pub fn remaining_daily_tokens(&self) -> Option<u64>

    /// Session token usage ratio in [0.0, ∞). ≥ 1.0 signals overrun.
    pub fn session_token_usage_ratio(&self) -> Option<f64>

    /// Daily cost usage ratio in [0.0, ∞). (mirrors session_cost_usage_ratio)
    pub fn daily_cost_usage_ratio(&self) -> Option<f64>
}

All methods return None when the corresponding limit is not configured,
matching the existing cost-method contract exactly.

Design Notes

• remaining_*_tokens uses saturating_sub so it always returns 0
  rather than panicking on overrun (u64 arithmetic).
• session_token_usage_ratio returns a value > 1.0 on overrun, which
  lets callers distinguish "near limit" from "already exceeded" without a
  separate is_exceeded() call.

Tests added

Test	Covers
test_budget_status_token_remaining_methods	Basic remaining calculation
test_budget_status_token_usage_ratio	Ratio at 25 %
test_budget_status_token_remaining_clamps_to_zero	Overrun → 0 remaining, ratio > 1.0
test_budget_status_no_token_limits_returns_none	No limits → None

cargo test -p mofa-kernel budget::tests

Checklist

• API matches existing cost-method conventions
• All new methods have doc-comments
• 4 new tests cover happy-path, edge cases, and overrun
• Fixes [BUG] BudgetStatus missing token remaining/usage-ratio methods #1475