feat: implement minimum and maximum bet limits per event

[Jagadeeshftw]

Summary

Adds configurable minimum and maximum bet limits per event (or globally) so bets are bounded and markets stay fair and liquid. Admins can set global defaults or per-event limits; place_bet validates amounts and rejects out-of-range bets with clear errors.

Changes

Types (types.rs)

• BetLimits – Contract type with min_bet and max_bet (i128).

Storage & logic (bets.rs)

• Global limits – Stored under bet_limits_global; used when no per-event limits exist.
• Per-event limits – Stored in a map keyed by market ID; override global for that market.
• get_effective_bet_limits(env, market_id) – Resolves effective limits: per-event → global → default (MIN_BET_AMOUNT / MAX_BET_AMOUNT).
• set_global_bet_limits / set_event_bet_limits – Validate bounds (min ≤ max, within absolute min/max), then store.
• place_bet – Validates amount with validate_bet_parameters(env, market_id, ...) using effective limits.
• Rejections: Below min → InsufficientStake; above max → InvalidInput.

Validation (validation.rs)

• validate_bet_amount_against_limits(amount, limits) – Shared min/max check used by bets.

Events (events.rs)

• BetLimitsUpdatedEvent – admin, scope (market_id or "global"), min_bet, max_bet, timestamp.
• emit_bet_limits_updated – Emitted when global or per-event limits are updated.

Public API (lib.rs)

• set_global_bet_limits(env, admin, min_bet, max_bet) – Admin-only; sets global limits and emits event.
• set_event_bet_limits(env, admin, market_id, min_bet, max_bet) – Admin-only; sets per-event limits and emits event.
• get_effective_bet_limits(env, market_id) – Returns effective BetLimits for a market.

Tests (bet_tests.rs)

• Set global limits and place at exactly min and exactly max.
• Place below configured min / above configured max (expect panic with clear error).
• Per-event limits override global; place at event min; place below event min (expect panic).
• Unauthorized caller cannot set limits (expect panic).
• Invalid bounds: min > max, min below absolute floor, max above absolute ceiling (expect panic).

Security

• Only stored admin can set limits.
• Limits are constrained to absolute bounds (MIN_BET_AMOUNT..MAX_BET_AMOUNT).
• No new error enum variants (reuses InsufficientStake, InvalidInput).

Testing

• cargo build – success
• cargo test – 419 passed (including 9 new bet-limits tests)
• stellar contract build --verbose – success

Closes #243

[greatest0fallt1me]

LGTM!