Encode string before xxhash in sagemaker boto3 mixin (fixes xxhash 4.0 TypeError)

[pingsutw]

Why are the changes needed?

build-plugins (flytekit-aws-sagemaker) is failing on master:

>  hash = xxhash.xxh64(sorted_dict_str(updated_config)).hexdigest()
E  TypeError: Strings must be encoded before hashing

4 failed, 140 passed

xxhash 4.0 removed the implicit encoding of str inputs, so xxh64() on a
string now raises. The plugin pins xxhash unbounded (plugins/flytekit-aws-sagemaker/setup.py),
so CI picked up 4.x and every code path that computes an idempotence token broke.

Failing tests: test_call_with_idempotence_token,
test_call_with_truncated_idempotence_token,
test_call_with_truncated_idempotence_token_as_input,
test_call_hashes_normalised_integer_values_consistently.

What changes were proposed in this pull request?

Encode the config string as utf-8 before hashing:

hash = xxhash.xxh64(sorted_dict_str(updated_config).encode("utf-8")).hexdigest()

utf-8 is exactly what xxhash <4 did internally, so the digests are unchanged —
existing idempotence tokens (and the tokens asserted in the tests) stay the
same, and this works on both xxhash 3.x and 4.x. Pinning xxhash<4 in setup.py
would also make CI green, but it leaves the latent bug in place, so I fixed the
call instead.

Verified the digest is stable across versions:

xxhash 3.8.1:  xxh64(s).hexdigest()                  -> 8d9d5330fea0133b
xxhash 3.8.1:  xxh64(s.encode("utf-8")).hexdigest()  -> 8d9d5330fea0133b

How was this patch tested?

No new tests — this restores 4 existing tests that fail on master. Ran the
suite locally against xxhash 4.0.1:

# without the fix
4 failed, 6 passed in 0.72s

# with the fix
10 passed in 1.52s

The token assertions (2de338ec95ee0da6, 1a25e4bb988697a7, ce735d6a183643f1)
pass unchanged, confirming the fix does not alter existing hashes.

Check all the applicable boxes

• I updated the documentation accordingly. (n/a)
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.00%. Comparing base (5f18ae7) to head (49cba7c).

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HEAD has 3 uploads less than BASE

Flag	BASE (5f18ae7)	HEAD (49cba7c)
	16	13

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3462       +/-   ##
===========================================
- Coverage   76.27%   52.00%   -24.27%     
===========================================
  Files         217      329      +112     
  Lines       22943    29585     +6642     
  Branches     3036     3036               
===========================================
- Hits        17499    15387     -2112     
- Misses       4556    14069     +9513     
+ Partials      888      129      -759     

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