feat(sdk): explorer URL builder helpers and unit tests

[emarc99]

• Closes [Wave 7 4/10] Explorer URL builder tests for Sepolia and Stellar testnet #73

What & Why

Centralized block explorer URL builders in @oversync/sdk under src/explorers/index.ts and added comprehensive unit test coverage for Sepolia Etherscan and StellarExpert Testnet URLs, as well as placeholder handling for mainnet/public networks.

Adds the missing stellarAccountUrl helper alongside ethereumTxUrl, ethereumAddressUrl, stellarTxUrl, and stellarContractUrl. Includes robust isValidInput validation so empty, whitespace, null, or undefined inputs safely return null instead of generating malformed links.

Touched surface

• frontend/ — React + Vite bridge UI
• packages/sdk/ — shared TypeScript SDK
• coordinator/ — order book + REST/WS service
• resolver/ — community resolver runner
• relayer/ — legacy v1 listener / watchdog (changes here need extra scrutiny)
• contracts/ — Solidity v2 (HTLCEscrow, ResolverRegistry)
• soroban/ — Stellar Soroban contracts (oversync-htlc, oversync-resolver-registry)
• docs/ — documentation only
• CI / config (.github/workflows/, docker, env.example)

Settlement & refund semantics

• Bridge settlement semantics changed (claim path, timelock ordering, preimage handling, hashlock type, asset routing)
• Refund semantics changed (who can refund, who receives refunds, timelock values, refund-address pinning)
• Settlement-critical invariant changed (e.g. non-custodial guarantee, no-admin-escape-hatch, permissionless refund)
• None of the above — this PR cannot move, hold, or release user funds

Tests run

• pnpm --filter @oversync/sdk build && pnpm --filter @oversync/sdk exec tsc --noEmit
• pnpm --filter @oversync/sdk test
• pnpm --filter @oversync/coordinator exec tsc --noEmit && pnpm --filter @oversync/coordinator test
• pnpm --filter @oversync/resolver exec tsc --noEmit && pnpm --filter @oversync/resolver test
• pnpm --filter @oversync/frontend exec tsc --noEmit && pnpm --filter @oversync/frontend test
• pnpm --filter @oversync/contracts compile && pnpm --filter @oversync/contracts exec hardhat test test/v2/HTLCEscrow.test.ts test/v2/ResolverRegistry.test.ts
• cd soroban && stellar contract build && cargo test --release
• (cd contracts && forge test --match-path "test/foundry/*" -v) (Solidity fuzz / invariant)
• pnpm test:e2e (cross-chain differential harness)
• node scripts/verify-addresses.mjs — required if addresses, configs, or env.example change
• node scripts/check-evidence-links.mjs — advisory; required if docs links change

Free-form outcome:

npx vitest run packages/sdk/test/explorers.test.ts

UI / evidence artefacts

• Frontend visible change → screenshot or short clip attached (swap flow, refund dialog, history banner, wallet confirm)
• Coordinator API change → curl snippet + JSON response sample pasted below
• Metrics / KPI change → updated snapshot in docs/examples/metrics-snapshot.example.json (or new JSON in PR)
• Status table / README change → updated row(s) pasted below with the source link
• None of the above — no UI-observable artefact

Secrets, logging, and PII risk

• No secrets, private keys, RPC credentials, .env content, wallet mnemonics, or preimages added to the repo
• No new console.* / logger.* line that prints secrets, preimages, signed payloads, or PII
• No new Vite/build flag that exposes devtools output in production (the VITE_* and esbuild.drop policy still holds)
• None of the above — pure string helper functions and unit tests only.

Public proof links (SCF / investor evidence)

n/a

Breaking change & rollback

• Breaking change? No — fully backwards-compatible additions to pure SDK helpers.
• Migration or feature flag required? No.

Reviewer checklist (for the PR author to self-verify)

• PR description and code comments are in English
• Linked issue or milestone ([Wave 7 4/10] Explorer URL builder tests for Sepolia and Stellar testnet #73)
• No unrelated drive-by changes (reformatting, dep bumps, etc.)
• Tests touch the same files as the source change
• PR is reversible: a single git revert restores prior state

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[emarc99]

@karagozemin kindly review whenever possible.

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@emarc99 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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