Add formatAddress utility for truncating Stellar addresses

[Kingsman-99]

Description

Stellar public keys are 56 characters long. Throughout the SDK, addresses are passed as raw strings with no helper to produce a display-friendly truncated form (e.g. GABCD...WXYZ). This issue adds a formatAddress utility to src/utils.ts.

Acceptance Criteria

• formatAddress(address: string, opts?: { leading?: number, trailing?: number }): string exported from src/utils.ts
• Default: first 5 characters + ... + last 4 characters → "GABCD...WXYZ"
• leading and trailing options override the defaults
• Throws SdkError with code INVALID_RECIPIENT if the address is shorter than leading + trailing + 3
• formatAddress exported from src/index.ts
• Unit tests: default format, custom leading/trailing, too-short address throws, full-length valid address formats correctly

Context

• Target files: src/utils.ts, src/index.ts
• Uses SdkError from src/errors.ts
• No new dependencies

[Peculiarr7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. I’ll prioritize this immediately and ensure a reliable solution is delivered as soon as possible. Thank you!

[bobo-24]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[afeezorobsco-cyber]

I would like to work on this issue

[adeshi105]

id like to work on this issue

[grantfox-oss]

🦊 GrantFox — @adeshi105 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #608)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@adeshi105's PR #633 was approved and merged by @Kingsman-99.

🏆 @adeshi105: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (271 total points). Track your full progress on GrantFox.

👏 Great work, @adeshi105! Keep contributing to Stellar-split.