feat: upgrade to stellar protocol 27

[zachfedor]

• Upgrade entire stack to Protocol 27: stellar-cli v27, soroban-sdk 27.0.0-rc.1, stellar-xdr 27.0.0, stellar-rpc-client 27.0.0
• Fix CI: update stellar/stellar-cli action to v27.0.0 in both build and test jobs
• Remove admin-sep dependency for test fixtures; rewrite hello_v1/hello_v2 fixtures with minimal inline upgrade logic
• Fix soroban-cli v27 breaking API changes

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	cargo/​tokio@​1.52.1 ⏵ 1.52.3
	cargo/​soroban-sdk@​23.5.2 ⏵ 27.0.0-rc.1	-1	+2
	cargo/​serde_json@​1.0.149 ⏵ 1.0.150
	cargo/​stellar-xdr@​26.0.0 ⏵ 27.0.0
	cargo/​soroban-spec-tools@​26.0.0 ⏵ 27.0.0

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[socket-security]

Warning

Review the following alerts detected in dependencies.

According to your organization's Security Policy, it is recommended to resolve "Warn" alerts. Learn more about Socket for GitHub.

Action	Severity	Alert  (click "▶" to expand/collapse)
Warn		Obfuscated code: cargo tokio is 90.0% likely obfuscated Confidence: 0.90 Location: Package overview From: Cargo.lock → cargo/tokio@1.52.3 ℹ Read more on: This package | This alert | What is obfuscated code? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Packages should not obfuscate their code. Consider not using packages with obfuscated code. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore cargo/tokio@1.52.3. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.

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[chadoh]

Nice work here, @zachfedor! This is way out of your typical wheelhouse and is some error-prone and frustrating work, but it looks like you found a way to make it less error-prone! 👏🏼 👏🏼 👏🏼

I'd love a couple quick confirmations from @willemneal on my questions below before we merge.