fix(platform): fix prod Sentry errors and reduce on-call alert noise (#12560)

## Summary
Hotfix targeting master for production Sentry errors that are triggering
on-call pages. Fixes actual bugs and expands Sentry filters to suppress
user-caused errors that are not platform issues.

### Bug Fixes
- **Workspace race condition** (`get_or_create_workspace`): Replaced
Prisma's non-atomic `upsert` with find-then-create pattern. Prisma's
upsert translates to SELECT + INSERT (not PostgreSQL's native `INSERT
... ON CONFLICT`), causing `UniqueViolationError` when concurrent
requests hit for the same user (e.g. copilot + file upload
simultaneously).
- **ChatSidebar crash**: Added null-safe `?.` for `sessions` which can
be `undefined` during error/loading states, preventing `TypeError:
Cannot read properties of undefined (reading 'length')`.
- **UsageLimits crash**: Added null-safe `?.` for
`usage.daily`/`usage.weekly` which can be `undefined` when the API
returns partial data, preventing `TypeError: Cannot read properties of
undefined (reading 'limit')`.

### Sentry Filter Improvements
Expanded backend `_before_send` to stop user-caused errors from reaching
Sentry and triggering on-call alerts:
- **Consolidated auth keywords** into a shared `_USER_AUTH_KEYWORDS`
list used by both exception-based and log-based filters (previously
duplicated).
- **Added missing auth keywords**: `"unauthorized"`, `"bad
credentials"`, `"insufficient authentication scopes"` — these were
leaking through.
- **Added user integration HTTP error filter**: `"http 401 error"`,
`"http 403 error"`, `"http 404 error"` — catches `BlockUnknownError` and
`HTTPClientError` from user integrations (expired GitHub tokens, wrong
Airtable IDs, etc.).
- **Fixed log-based event gap**: User auth errors logged via
`logger.error()` (not raised as exceptions) were bypassing the
`exc_info` filter. Now the same `_USER_AUTH_KEYWORDS` list is checked
against log messages too.

## On-Call Alerts Addressed

### Fixed (actual bugs)
| Alert | Issue | Root Cause |
|-------|-------|------------|
| `Unique constraint failed on the fields: (userId)` |
[AUTOGPT-SERVER-8BM](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BM)
| Prisma upsert race condition |
| `Unique constraint failed on the fields: (userId)` |
[AUTOGPT-SERVER-8BK](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BK)
| Same — via `/api/workspace/files/upload` |
| `Unique constraint failed on the fields: (userId)` |
[AUTOGPT-SERVER-8BN](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BN)
| Same — via `tools/call run_block` |
| `Upload failed (500): Unique constraint failed` |
[BUILDER-7GA](https://significant-gravitas.sentry.io/issues/BUILDER-7GA)
| Frontend surface of same workspace bug |
| `Cannot read properties of undefined (reading 'length')` |
[BUILDER-7GD](https://significant-gravitas.sentry.io/issues/BUILDER-7GD)
| `sessions` undefined in ChatSidebar |
| `Cannot read properties of undefined (reading 'limit')` |
[BUILDER-7GB](https://significant-gravitas.sentry.io/issues/BUILDER-7GB)
| `usage.daily` undefined in UsageLimits |

### Filtered (user-caused, not platform bugs)
| Alert | Issue | Why it's not a platform bug |
|-------|-------|-----------------------------|
| `Anthropic API error: invalid x-api-key` |
[AUTOGPT-SERVER-8B6](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8B6),
8B7, 8B8 | User provided invalid Anthropic API key |
| `AI condition evaluation failed: Incorrect API key` |
[AUTOGPT-SERVER-83Y](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-83Y)
| User's OpenAI key is wrong (4.5K events, 1 user) |
| `GithubListIssuesBlock: HTTP 401 Bad credentials` |
[AUTOGPT-SERVER-8BF](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BF)
| User's GitHub token expired |
| `HTTPClientError: HTTP 401 Unauthorized` |
[AUTOGPT-SERVER-8BG](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BG)
| Same — credential check endpoint |
| `GithubReadIssueBlock: HTTP 401 Bad credentials` |
[AUTOGPT-SERVER-8BH](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BH)
| Same — different block |
| `AirtableCreateBaseBlock: HTTP 404 MODEL_ID_NOT_FOUND` |
[AUTOGPT-SERVER-8BC](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-8BC)
| User's Airtable model ID is wrong |

### Not addressed in this PR
| Alert | Issue | Reason |
|-------|-------|--------|
| `Unexpected token '<', "<html><hea"...` |
[BUILDER-7GC](https://significant-gravitas.sentry.io/issues/BUILDER-7GC)
| Transient — backend briefly returned HTML error page |
| `undefined is not an object (activeResponse.state)` |
[BUILDER-71J](https://significant-gravitas.sentry.io/issues/BUILDER-71J)
| Bug in Vercel AI SDK `[email protected]`, already resolved |
| `Last Tool Output is needed` |
[AUTOGPT-SERVER-72T](https://significant-gravitas.sentry.io/issues/AUTOGPT-SERVER-72T)
| User graph misconfiguration (1 user, 21 events) |
| `Cannot set property ethereum` |
[BUILDER-7G6](https://significant-gravitas.sentry.io/issues/BUILDER-7G6)
| Browser wallet extension conflict |
| `File already exists at path` |
[BUILDER-7FS](https://significant-gravitas.sentry.io/issues/BUILDER-7FS)
| Expected 409 conflict |

## Test plan
- [ ] Verify workspace creation works for new users
- [ ] Verify concurrent workspace access (e.g. copilot + file upload)
doesn't error
- [ ] Verify copilot ChatSidebar and UsageLimits load correctly when API
returns partial/error data
- [ ] Verify user auth errors (invalid API keys, expired tokens) no
longer appear in Sentry after deployment