feat(nl2sql): add GLM, Doubao, and Sakana Fugu providers (19 → 22)

[devopam]

Summary

Widens the built-in translate_nl_to_sql fleet from 19 to 22 OpenAI-compatible providers — a low-risk adoption win. Each is a one-line entry in the nl2sql._PROVIDERS registry (every derived lookup table updates automatically):

Provider	Base URL	Key env var	Default model
GLM (Zhipu AI / Z.ai)	https://api.z.ai/api/paas/v4	ZAI_API_KEY (→ ZHIPUAI_API_KEY)	glm-4.5-flash
Doubao (ByteDance / Volcengine Ark)	https://ark.cn-beijing.volces.com/api/v3	ARK_API_KEY	doubao-seed-1-6-251015
Sakana Fugu	https://api.sakana.ai/v1	SAKANA_API_KEY	fugu

Base URLs, key env vars, and default models were each verified against the vendor's own docs. Adoption caveat: Doubao requires the model to be activated in the Ark console before the id resolves (documented in the user-guide row).

Docs bumped 19→22 (README, tools.md, tour.md); three rows added to the user-guide.md provider table; release-notes-0.6.9.md left at 19 (that release genuinely shipped 19). Key-discovery test extended with the new deviating env vars (ZAI_API_KEY, ARK_API_KEY).

Roadmap linkage

Advances roadmap row: N/A — extends the existing NL→SQL provider registry (feature already shipped; this is a data-only expansion).

Checklist

• Tests added/updated first; suite passes locally (38 provider tests green)
• ruff, ruff format, and mypy src/mcpg pass
• CHANGELOG.md updated under [Unreleased]
• Roadmap row cited (N/A)
• No hand-edits to src/mcpg/_vendor/

🤖 Generated with Claude Code

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Generated by Claude Code

Summary by Sourcery

Expand the built-in NL→SQL provider registry with three additional OpenAI-compatible vendors and update docs, changelog, and tests to reflect the enlarged provider fleet.

New Features:

• Add GLM (Zhipu AI / Z.ai) as a built-in NL→SQL provider with documented base URL, default model, and API key env vars.
• Add Doubao (ByteDance / Volcengine Ark) as a built-in NL→SQL provider with its default model and API key env var.
• Add Sakana Fugu as a built-in NL→SQL provider with its default model and API key env var.

Enhancements:

• Update configuration discovery tests to cover the new NL→SQL providers and their vendor-specific env vars.
• Refresh README, tools reference, user guide, and tour docs to reflect the expansion from 19 to 22 built-in NL→SQL providers.
• Document the new providers and their configuration details in the changelog.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/unit/test_config.py" line_range="420-422" />
<code_context>
         ("GITHUB_TOKEN", "github"),  # deviates
         ("DEEPINFRA_TOKEN", "deepinfra"),  # deviates
         ("SAMBANOVA_API_KEY", "sambanova"),
+        ("ZAI_API_KEY", "glm"),  # deviates
+        ("ARK_API_KEY", "doubao"),  # deviates
+        ("SAKANA_API_KEY", "sakana"),
     ],
 )
</code_context>
<issue_to_address>
**suggestion (testing):** Add a test case for the legacy `ZHIPUAI_API_KEY` fallback env var for GLM

The GLM provider spec accepts both `ZAI_API_KEY` and the legacy `ZHIPUAI_API_KEY`. This parametrized test only covers `ZAI_API_KEY`. Please extend the parametrization to also include `("ZHIPUAI_API_KEY", "glm")` so the legacy env var behavior is covered and guarded against regressions.
</issue_to_address>

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[gemini-code-assist-2]

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