fix: index signature str union ty check

[Peefy]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

fixes #1804

2. What is the scope of this PR (e.g. component or file name):

• kclvm/sema/src/resolver/config.rs

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

• test/grammar/schema/index_signature/fail_12/main.k

[coveralls]

Pull Request Test Coverage Report for Build 12608537845

Details

• 64 of 92 (69.57%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 70.711%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
kclvm/sema/src/resolver/config.rs	64	92	69.57%

Files with Coverage Reduction	New Missed Lines	%
kclvm/sema/src/resolver/config.rs	1	83.52%
kclvm/sema/src/core/symbol.rs	3	53.5%

Totals
Change from base Build 12558826184:	0.02%
Covered Lines:	52849
Relevant Lines:	74739

---

💛 - Coveralls

[He1pa]

lgtm

[zong-zhe]

LGTM