fix(builder): validate dialect clauses and translate MySQL upserts

[cofin]

Validates locking and conflict clauses in both build() and to_statement() and normalizes dialect aliases. Unsupported clauses raise SQLBuilderError instead of being silently dropped. MariaDB shared locks use LOCK IN SHARE MODE; unsupported lock modifiers are rejected. Spanner's two SQL modes retain plain FOR UPDATE and native upserts, with PostgreSQL-mode assignment restrictions checked.

MySQL/MariaDB conflicts translate to ON DUPLICATE KEY UPDATE, including excluded-value references and DO NOTHING self-assignment. The rewrite preserves the original builder AST and rejects predicates/constraints it cannot preserve. Documentation explains unique-key and trigger differences; regression coverage and v0.63.0 notes are included.

Validation: make lint passed during review. Regression tests were added or updated, but no local test suite was run in this review. Hosted CI for the latest branch head is still pending.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.10345% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.26%. Comparing base (8547d5d) to head (0aba425).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
sqlspec/builder/_base.py	92.68%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #778      +/-   ##
==========================================
+ Coverage   76.96%   77.26%   +0.29%     
==========================================
  Files         487      488       +1     
  Lines       71929    72084     +155     
  Branches    10075    10115      +40     
==========================================
+ Hits        55361    55695     +334     
+ Misses      13149    13115      -34     
+ Partials     3419     3274     -145     

Flag	Coverage Δ
py3.10	76.49% <93.10%> (+0.05%)	⬆️
py3.11	76.50% <93.10%> (+9.91%)	⬆️
py3.12	76.49% <93.10%> (+0.04%)	⬆️
py3.13	66.68% <93.10%> (+0.08%)	⬆️
py3.14	66.22% <93.02%> (+0.09%)	⬆️
unit	67.01% <93.10%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sqlspec/builder/_locking.py	79.59% <100.00%> (+0.86%)	⬆️
sqlspec/builder/_select.py	64.21% <100.00%> (ø)
sqlspec/core/hashing.py	85.96% <ø> (ø)
sqlspec/data_dictionary/_types.py	92.40% <100.00%> (+0.01%)	⬆️
...qlspec/data_dictionary/dialects/bigquery/config.py	100.00% <ø> (ø)
...pec/data_dictionary/dialects/cockroachdb/config.py	100.00% <ø> (ø)
sqlspec/data_dictionary/dialects/duckdb/config.py	100.00% <ø> (ø)
sqlspec/data_dictionary/dialects/mssql/config.py	65.79% <ø> (ø)
sqlspec/data_dictionary/dialects/mysql/config.py	80.68% <ø> (ø)
sqlspec/data_dictionary/dialects/oracle/config.py	82.35% <ø> (ø)
... and 4 more

... and 21 files with indirect coverage changes

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