feat: add copy-on-write derivation to Configuration

[OmarAlJarrah]

Problem

Configuration is immutable and built once via ConfigurationBuilder, but there was no supported way to take an existing instance and produce a slightly-different one. Callers wanting "the same configuration, but with LOG_LEVEL=DEBUG" had to reconstruct it from scratch and manually re-thread the env/property lookup seams — easy to get subtly wrong, and impossible for code that only holds a built Configuration (it can't see the original overrides or sources).

Change

Add copy-on-write derivation to Configuration, and round out its construction surface:

• Configuration.derive(Consumer<ConfigurationBuilder>) — derive a new Configuration by applying a mutator to a builder prefilled from the receiver, then building. The receiver is left untouched. This is the common derive-in-one-call path.
• Configuration.newBuilder() — returns the same prefilled builder, for callers that prefer to thread it through other builder-folding code before build().
• Configuration.builder() — @JvmStatic factory returning a fresh empty builder, matching the builder() idiom every other SDK model exposes (Request, Response, Headers, RequestConditions, RetrySettings, ClientIdentityStep, IdempotencyKeyStep) so Java callers no longer have to reach for new ConfigurationBuilder().
• ConfigurationBuilder now implements Builder<Configuration> and gains a prefilled constructor(Configuration), bringing it in line with the rest of the SDK's builders.

Together these complete the construction surface: builder() for from-scratch, newBuilder() / derive() for copy-on-write derivation from an existing instance.

Semantics are copy-on-write in the value sense: the override map is copied defensively so the original and derived instances never alias the same mutable state, while the envSource / propsSource lookup functions are shared by reference (they are pure read seams and are never mutated). A mutator that changes nothing yields an independent instance equal in behaviour to the original.

The method is named derive (rather than withOptions) because it names the operation directly — deriving a reconfigured copy — instead of introducing an "options" noun that appears nowhere else in the configuration API. newBuilder() (rather than toBuilder()) matches the prefilled-builder accessor every other model already exposes.

The three derivation read seams (overrides / envSource / propsSource) are exposed to the builder as @get:JvmSynthetic internal, so the prefilled constructor can read them without leaking a Java-callable accessor onto the public surface.

Tests

Cases covering: the builder() factory returning a fresh empty builder; adding a new override on the derived copy; the original staying unchanged; overriding an existing key without mutating the original; inherited env/property seams, with an explicit override winning over both an inherited property and an inherited env seam; seams shared by reference; source replacement inside the mutator detaching only the derived copy; the empty-mutator independent-but-equivalent case; empty-env skip-to-sysprop semantics surviving derivation; derivation from an override-less base; chained derivation accumulating overrides while each level stays independent; the null-mutator NullPointerException contract; and two builder-isolation cases (post-build mutation, and two independent derivations from one base not seeing each other's overrides).

API

Public-API additions; sdk-core.api snapshot regenerated via apiDump.

Gated build

./gradlew :sdk-core:test :sdk-core:apiCheck :sdk-core:ktlintCheck :sdk-core:detekt --no-daemon

Result: BUILD SUCCESSFUL.

Closes #60.