Avoid cloning Embassy mailbox values

[floze-the-genius]

Description

The Embassy mailbox push path cloned every value before its first try_send, even when the single slot was empty. Recover the rejected value from TrySendError::Full instead, drain the stale slot opportunistically, and retry with the same owned value.

The retry remains unconditional after a full result, preserving the race where a receiver drains the old item between the failed send and the overwrite attempt. Single-slot latest-value-wins behavior is unchanged.

A clone-counting regression verifies zero payload clones for both an empty-slot push and a full-slot overwrite.

Related Issue

• Closes Embassy adapter: eliminate the unconditional clone in Mailbox push #186

Verification

• Exact clone-counting regression: 1 passed
• Full aimdb-embassy-adapter: 16 unit, 3 connector, 1 session, and 4 doctests passed; 1 doctest ignored
• Strict adapter library clippy with -D warnings: passed
• cargo fmt -p aimdb-embassy-adapter -- --check: passed
• git diff --check: passed

The embedded thumbv7em-none-eabihf check could not run on this host because the installed Homebrew Rust toolchain has no rustup or embedded target (E0463: can't find crate for core). The repository-wide all-targets clippy also reaches an unrelated existing default_constructed_unit_structs lint in tests/session_smoke.rs:115; the changed library target is clean.

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding standards.
• I have added tests to cover my changes.
• All new and existing tests passed (make check).
• Documentation changes are not required for this internal performance fix.

[lxsaah]

lgtm!