feat: include server function id in the request URL

[birkskyum]

Closes #1795

Server function calls all went to a single _server URL, so access logs, traces and the network panel collapsed every function into one entry.

Requests now go to _server/<id>. In development the id ends with the source name, so you get POST /_server/13375fbc-0-ping with no configuration.

Production ids stay opaque by default. Opt in to keep the names:

solidStart({ serverFunctions: { readableIds: true } })

Notes:

• action.url changes from /_server?id=<id> to /_server/<id>. The handler still accepts the ?id= form, so hand-written URLs keep working.
• The route check was pathname.startsWith("/_server"), which also matched unrelated routes like /_serverless. It is now an exact match on /_server or a /_server/ prefix.

Tested: 12 new unit tests for the URL helpers, both e2e suites pass (30/30 dev, 30/30 bundled-dev), and verified against a production build that the new URL, the legacy ?id= URL, and readableIds all behave as expected.

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🦋 Changeset detected

Latest commit: f9b7211

The changes in this PR will be included in the next version bump.

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@solidjs/start	Minor

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[lxsmnsyc]

Hmmm this needs discussion. Pre-1.0 we had this kind of format then shifted to what we have today. I'm honestly fine with this change, but perhaps @ryansolid can weigh in on this

[birkskyum]

This issue has already waited for an opinion from Ryan for over a year... so it's not something we can hold back a release of Start 2 for. Closing for now.