feat: add automatic account fallback retry and bump version to 0.1.5

[gnahz77]

Summary
This PR improves multi-account request reliability by automatically retrying with another eligible OAuth account when the current request fails. It also bumps the package version to 0.1.5.
Changes

• add fetchWithAccountFallback to retry requests across eligible OAuth accounts
• expose resolveCandidateAccounts for candidate selection reuse
• update plugin request flow to prefer fallback-capable OAuth candidates when a raw model is specified
• keep existing winner-selection behavior by building resolveWinnerAccount on top of candidate resolution
• bump package version from 0.1.4 to 0.1.5
• update README and README.zh usage examples to reference 0.1.5
  Why
  Previously, a request could fail even when other authenticated accounts in the pool were still eligible for the target model. This change makes routing more resilient by failing over to the next available account automatically.
  Impact
• improves success rate for multi-account setups
• preserves existing account selection rules
• only applies fallback behavior to eligible OAuth accounts with refresh tokens

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