fix: auto-generate non-numeric id when linking JDK with numeric version

[maxandersen]

Problem

When a user tries to link an existing JDK using a numeric version, they hit a catch-22:

Attempt	Result
jbang jdk install 21 $JDK_PATH	❌ "id must be non-integer"
jbang jdk install java-21 $JDK_PATH then jbang --java java-21	❌ "--java only supports numeric"

The correct workaround (jdk install java-21 + --java 21) was not discoverable.

Fix

Instead of rejecting numeric IDs, auto-generate a non-numeric id using the pattern <version>-user:

$ jbang jdk install 21 /path/to/jdk
[jbang] Numeric id detected, using '21-user' as id
[jbang] JDK 21-user-linked has been linked to: /path/to/jdk

If 21-user-linked already exists, it increments: 21-user-1, 21-user-2, etc.

The linked JDK is then found by --java 21 via the version-based lookup in LinkedJdkProvider.

Tests

• Updated testJdkInstallWithLinkingAndIntegerId → verifies numeric id generates 11-user-linked
• Added testJdkInstallWithLinkingAndIntegerIdGeneratesIncrementedUserId → verifies counter suffix
• Updated testJdkInstallWithLinkingToExistingJdkPathWithDifferentVersion → verifies 13-user-linked

Fixes #2557

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[quintesse]

It's okay, although personally I wouldn't do the counter thing.
Because now, instead of being backwards compatible, we've introduced a new behaviour.
If before you would have run jbang jdk install 21 /foo/bar twice you would have gotten the message that 21 was already installed and to use --force if you really want to do that. With a counter you might end up installing more and more versions without really noticing. If you really want to install multiple 21s I think it should be an explicit act. (eg. the user explicitly giving it a unique name)

[maxandersen]

Good point. what prefix would you use for the one default ?

[quintesse]

Good point. what prefix would you use for the one default ?

I'm okay with the one you chose in the PR: "user" 👍

[quintesse]

LGTM