Test: pin both halves of the zip-slip entry-name contract

[AmaadMartin]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

Follow-up to google#621 (the zip-slip fix this pins). Original port: google#584.

2. Or, if no issue exists, describe the change:

Problem: The isDangerousZipEntryName suite asserts only rejection. Nothing asserts that a member name which merely contains .. inside a segment is still accepted, so an over-broad change stays green while it breaks archives with a benign dot-prefixed member. The docstring also still says the function mirrors adk-python's _load_skill_from_zip_bytes, which stopped being true at #621.

Solution: I pinned the accepted half of the contract with three new cases, and added the two bare dangerous shapes the committed list only covered in prefixed form. I rewrote the docstring to name the divergence and its reason, in the house style of the neighbouring isBareSkillName comment. No runtime behaviour changes: the source diff is comment lines only.

Notes for the reviewer:

• Parity rule applied. Parity wins for anything observable across the boundary; here the divergence is a rejection set, and adk-js rejects a strict superset of what adk-python rejects. No archive that adk-python accepts and extracts safely is broken by adk-js. adk-python is out of scope for this PR.
• The .. cases were appended to the existing it.each array. The assertion body, the test name and all five existing entries are unchanged, so no regression signal is lost.
• I did not add a/../../evil.txt. The committed references/../../esc.txt is the same shape and exercises no new branch.
• isDangerousZipEntryName stays module-private. The new tests drive it through loadSkillFromZipBuffer, as the existing cases do.
• Base sync. This fork's main was 13 commits behind google/adk-js main and did not contain Fix: reject a non-object Reasoning Engine raw body instead of crashing the dev server #621, so the new cases could not pass. I fast-forwarded the fork's main to upstream and rebased. The PR diff is two files.
• Collision check. gh pr list --repo AmaadMartin/adk-js --state open --limit 100 plus gh pr diff --name-only on every skills, zip and loader PR. Only Fix: key directory-loaded skill resources by POSIX relative paths #728 touches these two files. It rewrites resource keys in loadDir and adds tests in the loadSkillFromDir block; this PR touches isDangerousZipEntryName and the loadSkillFromZipBuffer block. There is no dependency between them, and Fix: key directory-loaded skill resources by POSIX relative paths #728 is based on a main that predates Fix: reject a non-object Reasoning Engine raw body instead of crashing the dev server #621, so I did not stack on it.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

npx vitest run --project unit:core core/test/skills/loader_test.ts — 52 passed, up from 47 on main.

Mutation proof. I ran each mutation against core/src/skills/loader.ts and confirmed the expected tests turn red.

#	Mutation	Result
1	segment === '..' -> segment.includes('..')	3 failed, 49 passed. The three new benign cases fail with Error: Dangerous zip entry ignored: references/..hidden.txt, ... assets/v1..2.bin, ... scripts/a..b.sh.
2	split(/[/\\]/) -> split('/')	2 failed, 50 passed: new case '..\evil.txt' and existing 'scripts\..\..\pwned.txt', both AssertionError: expected [Function] to throw an error.
3	some((segment) => segment === '..') -> startsWith('../')	5 failed, 47 passed, including the new case '..', all AssertionError: expected [Function] to throw an error.

Mutation 1 is the regression the new negative test exists to catch. It is green against main today.

Manual End-to-End (E2E) Tests:

No E2E surface. The change is a comment plus unit cases against an in-memory archive.

Other local checks on the pushed commit:

• npm run build — passes.
• npm run lint — clean.
• npm run format:check — clean.
• git diff main -- core/src/skills/loader.ts — every added and removed line is a comment line.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.