feat(agent): authenticate prepared mode

[jbachorik]

Summary

• bind prepared and dynamic control endpoints explicitly to loopback, including IPv4, IPv6, and port 0 discovery
• require an owner-protected token-file handshake before V1 or V2 negotiation in premain prepared mode
• discover the actual endpoint and credentials through the target JVM Attach properties for submit, list, and reconnect
• fail closed for missing, malformed, incorrect, symlinked, insecure, wildcard, and non-loopback configurations
• wipe in-memory token copies, remove generated credentials, clear discovery properties, and delay accept processing until initialization succeeds
• document the preamble, compatibility boundary, operator arguments, and credential lifecycle

Closes #874

Stack

This PR is stacked on #894. Review commit df70fda for the #874-only change.

Verification

• full :btrace-core:test suite
• full :btrace-client:test suite
• full :btrace-agent:test suite
• repository-wide spotlessCheck
• clean :btrace-dist:btraceJar
• full :btrace-dist:build
• focused V1/V2 authentication, malformed credential, timeout, IPv4/IPv6, token permission, symlink, startup noServer, and cleanup tests
• live prepared-mode distribution exercise: port 0 published a nonzero loopback endpoint and a 0600 token file; wrong credentials returned BTAF; the CLI authenticated, submitted and detached a probe, listed it, reconnected, and terminated it; target shutdown removed the generated token

The full agent suite currently needs a temporary local force of JUnit 5.14.4 because the develop baseline forces JUnit 6.1.2 into the Java 8 test compiler. That temporary build-file change was reverted and is not included here.

Adversarial review

• authentication is version-neutral and precedes construction of either command decoder
• authentication and negotiation reads are timeout-bounded on both peers
• V2-to-V1 fallback opens a new socket and reauthenticates; authentication failure never downgrades
• discovered endpoint updates are transactional and reject non-loopback metadata
• token parsing is bounded, avoids immutable secret strings, preserves token bytes, rejects multiple lines, and does not follow symlinks
• generated files use owner-only permissions and are deleted on failure or shutdown
• secrets are absent from arguments, system properties, logs, and exception messages

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This change is 

[jbachorik]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved the merge conflicts and pushed the merge commit: c1d0a1ae.