feat!: adopt numeric coercion contract

[toddbaert]

Adopts the numeric coercion ADR in the flagd provider.

• implement lossless numeric coercion in the in-process resolver (per-language flagd-core)
• for ResolveAll / AnyFlag.double_value, apply the same lossless check client-side so bulk eval matches individual Resolve* behavior
• bump flagd-testbed submodule; adopt @numeric-coercion-v2 and drop old @numeric-coercion exclusions/scenarios
• coordinated release with other providers
• breaking; call out in release notes

Covers both providers/flagd and the bundled providers/flagd-core (Java).

Parent: open-feature/flagd#1995

Depends on

Blocked until the following land first:

• feat!: implement numeric coercion contract flagd#1996 (flagd + flagd-core implementation)
• feat: add ADR-conformant numeric coercion scenarios under @numeric-coercion-v2 flagd-testbed#379 (@numeric-coercion-v2 scenarios and new testbed release)