fix: assign stmt in schema codegen and runtime value merge.

[Peefy]

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

• N
• Y

fix #785

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

• kclvm/compiler/src/codegen/llvm/node.rs
• kclvm/runtime/src/value/val_schema.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/assign_stmt/assign_stmt_0/main.k
• test/grammar/schema/assign_stmt/assign_stmt_1/main.k
• test/grammar/schema/assign_stmt/assign_stmt_2/main.k
• test/grammar/schema/assign_stmt/assign_stmt_3/main.k
• test/grammar/schema/assign_stmt/assign_stmt_4/main.k
• test/grammar/schema/assign_stmt/assign_stmt_5/main.k
• test/grammar/schema/assign_stmt/assign_stmt_6/main.k
• test/grammar/schema/optional_attr/inherit_2/main.k
• test/grammar/schema/optional_attr/inherit_3/main.k
• test/grammar/schema/init/init_nested_schema_1/main.k

[coveralls]

Pull Request Test Coverage Report for Build 6556537794

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.82%

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Totals
Change from base Build 6546481370:	0.0%
Covered Lines:	2300
Relevant Lines:	2619

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💛 - Coveralls