feat: upgrade error codes, Result-based error propagation, oracle price feed, and multi-token support

[khaadish]

Summary

This PR implements four contract improvements for the SkillSync Soroban smart contract:

#942 — Upgrade Error Codes

• Added InvalidWasmHash (600) and UpgradeFailed (601) error variants to ContractError
• The upgrade() function now validates that the provided WASM hash is not all zeros before attempting the upgrade

#943 — Error Propagation via Result Types

• Converted all public contract functions from panic_with_error! to return Result<T, ContractError>
• Introduced Result-returning helper functions:
  equire_initialized_result,
  equire_admin_result, get_session_result
• All error conditions now use the ? operator for clean propagation
• Updated tests to use ry_* methods for precise error code verification

#944 — Oracle Price Feed Module

• Added set_oracle(oracle_id: Address) admin function to register an oracle contract
• Added set_fallback_price(asset, price) for admin-provided fallback pricing
• Added get_asset_price(asset) that queries the oracle first and falls back to admin price
• Added get_cached_oracle_price(asset) and get_fallback_price(asset) view functions
• Oracle results are cached with timestamps for freshness tracking

#945 — Multi-Token Support

• Added oken_address: Option to the Session struct
• lock_funds now accepts an optional oken_address parameter
• When a token address is provided, the contract pulls tokens from the buyer via SEP-41 ransfer_from
• �pprove_session transfers tokens to the seller when a token is set

efund_session and �uto_refund return tokens to the buyer

• Sessions track which token is used; mixed-token sessions are prevented by design

Test Results

All existing tests updated and new tests added for:

• Upgrade error validation
• Error propagation (duplicate session, not found, amount validation, fee limits, not initialized)
• Oracle: set oracle, fallback price, price querying without oracle
• Multi-token: session creation with optional token address

Closes #942
Closes #943
Closes #944
Closes #945

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