fix(dd): reject oversized block sizes instead of crashing

[miniex]

Hello, I'm a developer from Korea, so sorry in advance if my English is a bit awkward 🙏

A very large block-size operand (ibs=, obs=, bs=, cbs=) makes dd crash instead of erroring out like GNU does:

• a value >= i64::MAX panics with attempt to multiply with overflow while the buffer size is computed (bug(dd): when ibs= is a very large number it panics #12844)
• a valid but very large obs aborts the whole process while allocating the output buffer (bug(dd): when obs= is a very large number it panics #12847)

# before
$ dd if=a of=b ibs=1778172772721772786161B
thread 'main' panicked at src/uu/dd/src/dd.rs: attempt to multiply with overflow
$ dd if=a of=b obs=1001111111772
memory allocation of 1001111111772 bytes failed
Aborted (core dumped)

# after
$ dd if=a of=b ibs=1778172772721772786161B
dd: invalid number: '1778172772721772786161B': Value too large for defined data type
$ dd if=a of=b obs=1001111111772
dd: IO error: out of memory

What I did:

• reject a block size >= i64::MAX at parse time, with the same "Value too large for defined data type" message as GNU
• use saturating_mul for the lcm buffer-size computation so it can't overflow
• allocate the output buffer with try_reserve, so a too-large obs fails gracefully instead of aborting

I also added regression tests. cargo fmt, clippy (-D warnings) and the dd test suite all pass on my machine (x86_64 Linux).

Fixes #12844
Fixes #12847

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[sylvestre]

why using unwrap here?

[HackingRepo]

unwrap will introduce panics again that is i see, which we want suspose to prevent we spent hours to search for panics, so please do'nt use panic functions handle the Result