Make BundledHTTPRequest's init checks hold, and add Equatable

[germ-mark]

Split out of #37, which had grown core-library changes inside a testing-mocks PR. This is independent of #32 and targets main.

init rejects a .get with a body and a url with no scheme, but both stored properties were publicly settable, so a caller could assign a body onto a .get, or flip the method of one that had a body, and reach a state init rejects. Validation was advisory rather than structural — which is also why the guard in URLSession.data(for:) is load-bearing rather than redundant.

body is now let and request is private(set). Header fields cannot invalidate either check, so settingHeader(_:for:) keeps them settable through a narrow API.

Two bugs found while writing the tests

• missingScheme was unreachable in debug. init called assert(false) before throwing it, so that path trapped rather than surfacing the error — and could not be tested at all. My first attempt at a test segfaulted the suite.

• urn: and mailto: urls produced requests that could never be sent. HTTPRequest drops everything after the scheme for those and reports url == nil. Anything comparing through request.url then called every url of that shape equal to every other:

  BundledHTTPRequest(url: URL("urn:isbn:1")!) == BundledHTTPRequest(url: URL("urn:isbn:2")!)
  observed: true
  

  init now rejects them with the new HTTPRequestError.unrepresentableURL. Equatable is synthesized — HTTPRequest is already Hashable — so it compares scheme, authority and path structurally rather than depending on that rejection staying in place.

Source-breaking

oauth4swift assigns header fields in place at three sites (DPoP/Signing.swift, and twice in Session/OAuthSession+AuthRequest.swift) and needs the companion migration to settingHeader(_:for:). I verified the break rather than assuming it: unfixed oauth4swift main against this fails with "'request' setter is inaccessible", and builds clean with the migration. A multiline-aware sweep of oauth4swift, AtprotoOAuth, AtprotoClient, GermShared, CoreAppLogic and ATResolve found no other mutator.

The companion commit is ready on oauth4swift's llm/adopt-germconvenience-api, waiting on this release. Note minor doesn't actually gate consumers on 0.x, since SwiftPM's from: is upToNextMajor — the lockstep release is what protects them.

Test notes

38 tests, 10 suites, clean build, no warnings. settingHeader is covered including the two behaviours it inherits from HTTPFields: repeated fields collapse to one comma-joined value, and a pseudo-header name traps.

Once this lands, #32's extension BundledHTTPRequest: Equatable in GermConvenienceMocks should be deleted — it would otherwise be a duplicate conformance. There's a comment marking it.

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