receive: treat an unset tenant_matcher_type as exact in shard size overrides

[SaiPisey2]

Changes

Fixes #8983.

getShardSize switched on the matcher type and only handled the explicit exact value:

switch override.TenantMatcherType {
case TenantMatcherTypeExact:
    ...
case TenantMatcherGlob:

An override that leaves tenant_matcher_type unset therefore matches no case and is skipped, so the tenant silently falls back to the default shard size — losing the routing determinism the override exists to provide.

Empty already means exact everywhere else in the package: it is documented on TenantMatcherTypeExact ("This is also the default one"), and isExactMatcher treats "" and "exact" alike for tenantSet.match. This uses that same helper so the two paths agree.

Verification

Added TestShuffleShardHashringGetShardSize, covering an override with no matcher type, one with an explicit exact, a glob, and a tenant with no override. The unset case returns 2 instead of 3 on main and passes here.

Note three tests fail on main in my environment (darwin/arm64), unrelated to this change and present before it: TestRelabelWithUnsetValidationScheme, TestKetamaHashringEvenNodeSpread and writecapnp.TestMarshalWriteRequest.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.57%. Comparing base (bc87f18) to head (0460f48).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9003   +/-   ##
=======================================
  Coverage   64.57%   64.57%           
=======================================
  Files         289      289           
  Lines       37410    37410           
=======================================
  Hits        24156    24156           
  Misses      11155    11155           
  Partials     2099     2099           

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[SaiPisey2]

Rebased onto main. The conflict was only the changelog, where #9014 landed in the same ### Fixed block; both entries are kept, in PR order.

26 checks pass. Ready for review whenever someone has a moment.